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

9$\begingroup$ Coincidentally or not, an article on mathematical sculpture just came out in the AMS Notices: ams.org/notices/201007/rtx100700840p.pdf $\endgroup$– Charles StaatsJul 19, 2010 at 13:24

8$\begingroup$ There is absolutely no reason to close this beautiful (literally) question. $\endgroup$– Gil KalaiNov 14, 2013 at 21:13
43 Answers
Most of the artistic work of the Italian Attilio Pierelli was inspired by mathematics and notably by the idea of representing the fourth dimension. You can see some pictures of his "hyperspaces" at his site: www.pierelli.it/
I've always liked Robert Engman's sculptures
Miguel Berrocal.
http://www.berrocal.net/index_eng.html
I also remember a beautiful article by Martin Gardner on the mathematical aspects of his sculpture.
George Green's windmill in Nottingham has a sculpture with a mill wheel with Green's Theorem carved into it.

$\begingroup$ Background: "George Green, Mathematician and Physicist 17931841" by Cannell and Lord (jstor.org/stable/3619259), and "From Grindstones to Theorems: The History of Green's Mill" by Ian Douglas (timetravelbritain.com/articles/museums/greensmill.shtml). $\endgroup$ Nov 9, 2020 at 16:25
Sugimoto Hiroshi has made some beautiful mathematical surfaces. They are described in his book Conceptual Forms and also in his web site. I have seen some of them "live" in the museum island of Naoshima.
Jane and John Kostick make many mathematically inspired sculptures some of which can be seen here: http://www.jjkostick.com/jjkostick/Welcome.html
For example, Jane made a coffee table whose base is a trefoil knot.
For two more examples of sculptures that Jane built, please see the December 2008 issue of the Girls' Angle Bulletin, which can be downloaded from: http://www.girlsangle.org/page/bulletin.html
I think there are some sculptures by Helaman Ferguson on the campuses of Macalester College and the University of St. Thomas, both in St. Paul, Minnesota.
Perhaps these are too small to count as sculptures, but there is quite a respectable collection of models of mathematical objects on display in the Mathematical Institute in Goettingen.
Credit to the photographer for this shot of Charles O. Perry's 'Solstice' in my downtown Tampa (a '2/3twist triangular torus Mobius strip' according to him in this article about his work)
Frank Stella's Alu Truss Star (gallery website) is currently in the sculpture garden of the New Orleans Museum of Art. I believe it's a stellated dodecahedron, about 14 feet tall. I spent a few minutes looking at it from different angles to observe its symmetry when I ran across it in situ.
Hyperboloids! Not necessarily a famous sculpture but very nice to look at. I have seen them everywhere from math classrooms to gardens!!
Kryptos, Jim Sanborn, Langley Virginia USA
https://www.wired.com/2013/07/nsacrackedkryptosbeforecia/
Sculpture by Nina Douglas, at the Simons Center for Geometry & Physics
(There is also a copy at IHES)