Either intentionally or unintentionally. Include location and sculptor, if known.
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.
I've always liked Robert Engman's sculptures
I also remember a beautiful article by Martin Gardner on the mathematical aspects of his sculpture.
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
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