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

The Found Math galleries at the MAA website include many mathematical sculptures. 


George Green's windmill in Nottingham has a sculpture with a mill wheel with Green's Theorem carved into it. 


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. 


The french sculptor Bernar (sic) Venet: see http://images.math.cnrs.fr/BernarVenetdelartetdes.html 


Among several of his sculptures, personally for me the neatest one  (it's at the Newton Institute) 

