Either intentionally or unintentionally. Include location and sculptor, if known.

8$\begingroup$ Coincidentally or not, an article on mathematical sculpture just came out in the AMS Notices: ams.org/notices/201007/rtx100700840p.pdf $\endgroup$ – Charles Staats Jul 19 '10 at 13:24

6$\begingroup$ There is absolutely no reason to close this beautiful (literally) question. $\endgroup$ – Gil Kalai Nov 14 '13 at 21:13
The Mathematical Research Institute in Oberwolfach have a sculpture on their grounds depicting Boy's surface:

$\begingroup$ It was a gift of the MercedesBenz... but is there an artist who made it? $\endgroup$ – Pietro Majer Jul 19 '10 at 13:47

$\begingroup$ I don't read German, perhaps in the linked article here mfo.de/general/boy there's an answer? $\endgroup$ – Willie Wong Jul 19 '10 at 13:50

$\begingroup$ The article seems to say that it was produced by a computeraidedmanufacturing system at MercedesBenz, rather than an artist. $\endgroup$ – user5117 Jul 20 '10 at 10:01

3$\begingroup$ so, remarkably, the most appeciated sculpture here was made by a computer? $\endgroup$ – Pietro Majer Jul 20 '10 at 16:34

33$\begingroup$ Well, many sculptures, even when "made by an artist" were in fact made after the artist programmed it into a computer, which then did the work. We don't say that other works were "made by a chisel" or "made by a paintbrush" do we? $\endgroup$ – Gerald Edgar Mar 13 '11 at 13:21
The centrepiece of McAllister building, which houses the math department at Penn State, is the Octacube, designed by Adrian Ocneanu.
There's a bit of a description of the mathematics behind the Octacube on the Penn State website, but unfortunately, that's the most material that I can find online. There are all kinds of animations set up to display on a computer terminal in McAllister building, but they don't seem to be available online anymore.
Very briefly, the mathematics of the sculpture is as follows: consider the fourdimensional regular convex polytope whose vertex set is the union of the vertex sets for the fourdimensional cube {(±1,±1,±1,±1)} and the fourdimensional octahedron {(±2,0,0,0), (0,±2,0,0), (0,0,±2,0), (0,0,0,±2)}. Consider the 1skeleton of this polytope (vertices and edges), and project radially to S^{3} ⊂ ℝ^{4}. Project the resulting "inflated polytope" stereographically to ℝ^{3}, and "fatten" the edges so that a crosssection of an edge is no longer just a point, but a Yshape (see the corners of the sculpture). What you get is the sculpture shown.

$\begingroup$ Do you mean "project radially to $S^3$"? $\endgroup$ – Charles Staats Jul 19 '10 at 18:53

$\begingroup$ Oops... I absolutely do. Fixed it in the post... thanks for the correction. $\endgroup$ – Vaughn Climenhaga Jul 19 '10 at 19:23
Helaman Ferguson. My department has one of these in the main office:
Keizo Ushio. He made this during the 2006 ICM in Madrid:
_{(source: memenet.or.jp)}

1$\begingroup$ He's got some nice sculptures at MSRI, too. $\endgroup$ – Allen Knutson Oct 26 '10 at 0:54

$\begingroup$ There is also a paper, Friedman and Sequin, Keizo Ushio’s Sculptures, Split Tori and Möbius Bands, available at maths.ed.ac.uk/~aar/papers/keizo.pdf $\endgroup$ – Gerry Myerson Nov 10 '13 at 23:09
MoMath, in partnership with Make, has a regular feature on DIY math sculptures. I personally like the space filling curve made of steel pipe ells by Chaim GoodmanStrauss and Eugene Sargent.
Adding to the list two of my favorite mathematical sculptors:
George Hart:
http://www.georgehart.com/sculpture/sculpture.html
Bathsheba:
Finally, there are a lot of nice things at the new Geometry Playground in the Exploratorium, for anyone coming through the SF bay area:

$\begingroup$ Comments about and samples of George Hart's fascinating work can be found here: richbugger.wordpress.com/2009/12/04/… $\endgroup$ – Joseph Malkevitch Jul 20 '10 at 1:09

$\begingroup$ +1 for George Hart; I had the pleasure of taking a few of his classes at Stony Brook. He's a great teacher, and a great person. Also, mathematical sculptures are his life. $\endgroup$ – BlueRaja Jul 20 '10 at 6:04
A month ago our team completed, over four days, a very large geometric sculpture out of 20 tons of snow. Eva Hild of Sweden designed it and came over to work with us. The complete story is at
http://stanwagon.com/snow/breck2011/index.html
The videos linked at the top of the page allow you to walk around the work.
Stan Wagon

$\begingroup$ Wow! Very impressive. Shame that it will probably be left to melt (although the evolution of that might also be cool). $\endgroup$ – Todd Trimble♦ Feb 8 '14 at 12:47
Models at the University of Regensburg Math Dept. An example,
Polyhedron sundials by Stefano Buonsignori (16th century) in the Medici collection presented by Museo Galileo. The 3D permutahedron:
(The combinatorics underlying the geometry of the permutahedra are related to multiplicative inversion and, consequently, to Koszul duality and the formalism of Appell sequences and associated matrices, so I find this particular jewel at the juncture of art, engineering, geometry, combinatorics, and analysis particularly exciting. If only he had done one for the associahedron!)
CalabiYau: the shape of the inner space of the universe. In Yau's hometown in south China.
Models in resin made by Stewart Dickson of clay models made by Bernard Morin (a blind topologist!) of different stages of the eversion of a sphere. The resin models were presented at the Maubeuge symposium "Arts et Mathématiques." (Photo by John Sullivan.)
A sculpture by Gideon Weisz of an approximation of Alexander's horned sphere to five levels.
(Photo from “The Universal Book of Mathematics: From Abracadabra to Zeno's Paradoxes” by David Darling.)

$\begingroup$ For more info on the instrument, see hsm.stackexchange.com/questions/5573/…. $\endgroup$ – Tom Copeland Jan 14 '17 at 0:49

$\begingroup$ On the horned sphere youtube.com/watch?v=d1Vjsm9pQlc $\endgroup$ – Tom Copeland Feb 22 at 0:47
I suspect that the Octacube is also the only mathematical sculpture (possibly the only mathematical topic?) to appear in the journal Playboy (March 2006).
Alessandro Giorgi has made a bunch of statues concerned with the mathematics of juggling, e.g. 1 and 2.

$\begingroup$ It seems a bit misleading to have the balls attached to circles. Real juggling balls follow parabolic trajectories...! $\endgroup$ – Tobias Fritz Nov 15 '13 at 14:50
at Ohio State ...
The Garden of Constants is a sculptural garden of large numerals that highlight the activities performed in nearby College of Engineering buildings. The installation, by Barbara Grygutis, includes a black walkway featuring 50 individual formulas cast in bronze and embedded in handmade pavers. The Garden of Constants is on the lawn of Dreese Laboratories.

$\begingroup$ That settles the dispute whether 1 is prime! $\endgroup$ – Victor Protsak Jul 20 '10 at 3:36

1$\begingroup$ Victor, there are other numbers there besides 1 and primes, actually. They're just out of view. $\endgroup$ – KConrad Jul 20 '10 at 4:50

2

$\begingroup$ This particular work consists only of the ten digits. $\endgroup$ – Gerald Edgar Mar 13 '11 at 13:23

18
Borromean rings on campus of George Washington University in Washington, DC. Unfortunately, I didn't make a note of the name of the artist.

1

$\begingroup$ @KCo, I'll try to find it on my computer. Meanwhile, I found a pictur of it on the web. $\endgroup$ – Gerry Myerson Dec 16 '18 at 8:33
A ruled surface seen at the Peggy Guggenheim collection in Venice (I believe it is by Antoine Pevsner):

$\begingroup$ That's really nice. Do you happen to know the medium it's done in? I think my mother (who is a sculptor) would be intrigued. $\endgroup$ – Todd Trimble♦ Feb 8 '14 at 12:45

$\begingroup$ @ToddTrimble: Hi Todd, it had a "coppery" look, but not sure if it was mixed with anything else. $\endgroup$ – auniket Feb 19 '14 at 13:50
Here are some sculptures by Henry Segerman: https://plus.google.com/u/0/photos/102006004474081559466/albums/5946201022131183089
I think tensegrity sculptures are cool. Kenneth Snelson does a lot of these.
Cliff Stoll makes Klein bottles and sells them too. He's made the "worlds biggest klein bottle". You can see that here.
I like the 4 dimensional mathematical sculptures by Bathsheba Grossman, such as the 24cell:
http://www.bathsheba.com/math/24cell/24cell_new_th.jpg
Are the cryptographic sculptures by Jim Sanborn  Cyrillic Projector, etc.  close enough to a "mathematical sculpture"?

2

The sculptures of Morton C Bradley are not so widely known. He was originally an art conservator in Boston, but he also explored geometric shapes and color to create many sculptures. They are "a reflection of his fascination with the science of color, his admiration for traditional patterns, his exploration of mathematical designs." (Quote from web site below.) He never sold a single piece of work. At his death he donated his entire estate to Indiana University. The IU Art Museum is cataloging his work and arranging exhibits. They've created a web site http://www.iub.edu/~iuam/online_modules/bradley/ that discusses and displays some of his work. In comparison with other work already mentioned here, perhaps the most significant thing is his deep exploration of color in his sculptures.
I am by coincidence in Paris at this moment, attending a meeting (the first) of ESMA, the European Society for Math and Art. George Hart is giving a talk in two hours on his sculpture. The website of ESMA is here: http://mathart.eu/ and the program of the meeting here: http://mathart.eu/en/conf10program.html, and a web tour of the associated exhibition is here: http://mathart.eu/ihp10/index.html There was a talk yesterday about Stan Wagon's snow sculptures by John Sullivan, one of the team.
Stan Wagon (with various collaborators) is known to make snow sculptures that are mathematically pleasant.
Among several of his sculptures, personally for me the neatest one 
(it's at the Newton Institute)
Check these out...they spring into shape by creating parallel folds on an initial paper shape:
http://erikdemaine.org/curved/
Erik Demaine is a mathematician (computational geometer) at MIT.

4$\begingroup$ One cannot mention Erik Demaine without also noting Robert Lang. langorigami.com $\endgroup$ – Willie Wong Jul 19 '10 at 13:00

"Tucker's group of genus 2" by Duane Martinez and DeWitt Godfrey
http://www.colgate.edu/news/blog/archives/archivedisplay?nwID=5031
http://commons.wikimedia.org/wiki/File:Tucker%27s_Genus_Two_Group.jpg

$\begingroup$ The Colgate image, and the Wikipedia article and image, have all been deleted. However I kept an image of part of the sculpture (a detail of a photograph, though it looks like a computergenerated image) at weddslist.com/groups/genus/g2/sculpture.jpg . The physical sculpture is in Slovenia, in this museum: burger.si/Bistra/BistraENG.html $\endgroup$ – maproom Nov 11 '13 at 16:53
Does the National Aquatics Centre in Beijing count? It illustrates the WeairePhelan structure, a recent (and the first) counterexample to Kelvin's conjecture.
Here's one of a large number of nice mathematical sculptures at the Science Museum in London.
This sculpture is made of 30 interlocking pieces and requires many hands to assemble or disassemble.
This sprinkler illustrates a theorem that a 3d solid can be constructed to cast any desired set of silhouettes (also illustrated on the cover of Gödel, Escher, Bach).
The last three were found on the threedimensional geometry page of this wonderful collection.
Finally, a shameless plug of one of my own questions.
The french sculptor Bernar (sic) Venet: see http://images.math.cnrs.fr/BernarVenetdelartetdes.html

$\begingroup$ "Commutative Operation, 2001" made me laugh ;) $\endgroup$ – Piotr Achinger Feb 7 '14 at 18:22

$\begingroup$ I liked how the devotee of art was staring at "S Matrix Element, 2001". $\endgroup$ – Todd Trimble♦ Feb 8 '14 at 12:42
Most of the artistic work of the Italian Attilio Pierelli was inspired by mathematics and notably by the idea of representing the forth dimension. You can see some pictures of his "hyperspaces" at his site: www.pierelli.it/
The Found Math galleries at the MAA website include many mathematical sculptures.
There are a few lying around Fine Hall in Princeton. One I think is "Five Disks: One Empty" by Alexander Calder.
There's also Marc Pelletier's "Polydodecahedron" which was gifted to JH Conway. http://www.princetonoccasion.org/quarkpark/pages_statements/Conway.html
There's also this big Obelisk in the common room (or did it get moved recently? I seem to remember its relocation when they redid the flooring). I had forgotten what it is actually called and what mathematics it is supposed to represent. Someone should edit this to add in the details.