Just like Google dominates contextual search, one small company wishes to become the de facto search engine for 3D printable models. That's not all. They allow you to search by uploading similar models of what you may be looking for or by using other models already within their database. The site called, 3DShap.es launched only recently and is still in beta, but so far their search is pretty interesting. More details on this 3D model search engine can be found here: http://3dprint.com/10236/3dshapes-3d-model-search

Below is a screenshot of their main search page: