I ran across STL Finder and it’s a pretty cool site for finding 3D models for your 3D printer.  I wouldn’t call it a “search engine” in the classic sense in that it doesn’t appear to be indexing broad online content like Google does, but rather it is indexing files from major and submitted online repositories.  Like Kayak is for travel, it’s a fast way to search multiple file sites at once.  The site claims 2.5 million files and that’s pretty impressive!