We hope to find many more open sources for 3D designs, but the following list are the ones we have used in the Center.
Thingiverse – started by Makerbot Industries, this is the site for finding (and sharing!) ready-to-print designs.
Google 3D Warehouse – this site is a large repository for designs that are compatible with Google SketchUp. Most are probably not designed to be 3D-printed, but most are 3D and could probably be adapted.
sharecg – this site is most frequented by computer graphics designers and moviemakers. However, the objects are freely available and most come in a format that is convertible to something that can be printed.