I've pulled together various icon sets that are open source and free for use.

37 Signals

Description: Very small set of vector icons from here.
License: Completely free
Attribution Required?: No
Download from: http://tedwi.se/u/92

Silk

Description: Huge set of small 16x16 icons from here.
License: Creative Commons
Attribution Required?: Yes
Download from: http://tedwi.se/u/94

Crystal

Description: Large set of icons ranging from small sizes to high resolution from here.
License: LGPL
Attribution Required?: Yes
Download from: http://tedwi.se/u/95