Nice free icons set released under a Creative Commons Attribution (CC-BY) license.
A whopping 900 map icons, released under the GPLv3. Colour coded, categorised, and all sorts.
The icon from ShareThis.com and the ShareThis widget is somewhat free to use, subject to their guidelines (no use in trademarks, etc). They want it to replace all the individual social networking and other icons and be the de facto standard.
Quite a famous set of icons set released under a Creative Commons Attribution (CC-BY) license and a debian compatible license.