Finding good quality, consistently themed, free icon sets that can be used in commercial applications isn’t always easy.
Fortunately there are two available that which have pretty much all you will ever need.
- http://p.yusukekamiyamane.com/ – 3,361 icons in a variety of sizes
- http://www.famfamfam.com/ – a little older but the Silk set has 1000 icons
Both are available under the Creative Commons Attribution 3.0 License, which means you can use them for pretty much anything you like as long as you add a reference to the author in the readme.