These are some of the icon sets I've used in the past - generally Creative Commons - and always used with attribution.
For general use:
http://code.google.com/p/fugue-icons-src/ - preview at: http://p.yusukekamiyamane.com/icons/preview/fugue.png
- plus SyncFusion have a free set which is growing big now - http://www.syncfusion.com/downloads/metrostudio
- plus BootStrap from Twitter has some interesting icons built-in