If you need a really nice, large set of free icons to use in your web or desktop applications, I recommend the Fugue set. As of November 30, 2009, it has 2360 icons, includes the Adobe Photoshop PSD files in case you need to modify them and they are free if you provide a link back to the site from your application, or you can pay a fee of $50 and skip the link back.

Below is a sample of the icons: