I'm sure there are plenty of free icon sets on the net but since I always forget the few I find, this time I'm going to blog about it to not forget it.
The Silk Icons is a really nice set of 16x16 pixel icons free and free for commercial use. You can either download them all as individual PNG files or preview them all using this one big image file (1Mb)
Whilst on the subject it's also worth keeping the iconfinder in mind. It's in beta and both limited and slow but worth keeping an eye on.