Robin Good: The Icon Library offers over 350,000 free icons which can be sorted by category, artist, popularity, date or searched by color, tag or name.
Favorite icons can be easily collected, organized and shared with others and can be used across different operating systems.
The library exists since 1994 and it has been growing steadily, by collecting quality icon designs from capable artists and supporting the best ones by purchasing their icon licenses.
Good stuff and free to use. Recommended.
Check it out now: http://www.iconarchive.com/
Via Robin Good, Jimun Gimm, Rui Guimarães Lima, Kim Flintoff