This is a great CAD library, search tool, and library part builder. It works with most of the common electronic circuit CAD programmes. The search interface can be found here. I particularly like the quick component builder that lets you create your own library elements and then share them with other users. It has already saved me a great deal of time, as it is much faster creating new library parts for Eagle than using the component editor within Eagle, even when starting from a similar part already in Eagle.
For the uninitiated Eagle is the schematic capture and PCB design tool I use at Gollege.