We can all be a bit lazy at times (specially me), so I find it easier to simply drag and drop models into my assemblies rather than model them from scratch (I tell the boss it's more productive).
For that reason, I've started this thread as a ready reference for sites where good solid data can be easily accessed. Here are a few to start, please add some of your own.
Charles Bliss' Inventor Page
Sean Dotson's Part Repository
Kent Keller's Kwik MCad Site
Jan-Steinar Jakobsen's Site
Powerparts on Web
Yahoo Inventor Parts Forum
Please note - some of these sites require a free registration.