http://www.axialis.com/free/icons/
Nice free icons set released under a Creative Commons Attribution (CC-BY) license.
http://code.google.com/p/google-maps-icons/
A whopping 900 map icons, released under the GPLv3. Colour coded, categorised, and all sorts.
http://www.nicora.net/yh/webtoodotoh/default.html
A team of designers applied a Web 2.0 look and feel to a selection of well known corporate logos.
http://www.logolounge.com/articles/default.asp?ArticleID=540
Yearly logo trends from LogoLounge.com
http://www.famfamfam.com/lab/icons/silk/
Quite a famous set of icons set released under a Creative Commons Attribution (CC-BY) license and a debian compatible license.