I came across a great site today that has an immense collection of free, quality icon images that developers can use in their projects. The Silk collection is really cool!
They are published with a Creative Commons license, so as long as you give the guy credit and include a link to his site in your about box, you are free to use them any way you wish in your projects, including making derivative works of them.
They are in .png format but they can be easily converted to .bmp or .ico.
How can you go wrong?
So head on over to FamFamFam and check them out!