Like GrabCAD (http://grabcad.com/), This site allows you to download STLs from a large selection of user-made files. Unlike GrabCAD, you do not need to make an account to download the 'thing's. There seems to be quite a large selection of mathematical type of objects contained within the site. Beyond that, one is able to find a large assortment of 3D objects from jewelry to gadgets. One of the things interesting I found on here was the ability to edit some of the projects and the capabilities these projects had. For example, there was a project I found that was able to take any image you wanted to make 3D and place it within a 100x100 mm square frame. (http://www.thingiverse.com/thing:74322). Altogether, this site is a good go-to for quickly finding a variety of 3D printable objects.