- Silk Icons - under Creative Commons Attribution 2.5 License (use for any purpose, can modify, but have to credit author)
- Mini Icons - free for any purpose
- graphicPUSH: Free icons - under Creative Commons Attribution-ShareAlike 2.5 License (use for any purpose, can modify, but have to credit author and share modifications under same license).
- IconBuffet - free icons every month (registration required). You can also buy icons from them. No royalties to pay.
- Ganato - Free Icons - in PSD (Photoshop) format
- KDE-Look.org icons - under GPL (so not sure if that would mean the page they are on has to be as well, but I assume not as the format is different)
- Sweetie | Free icon set - under Creative Commons Attribution-ShareAlike 2.0 License
There is also a list that is updated @ MaxPower - Free! Icons for your website or application
There probably are more sites with free icons, which I will try to add if I find them. Tutorial on creating your own icons with GIMP: GIMP - Creating Icons.