FatCow Web Hosting generously provided 1000 "farm-fresh" icons that are useful in all situations, and comes in both 16 and 32 pixel sizes.
Download FatCow web icons here
LED Icons is another fantastic set of 512 icons with a wide range of uses that can easily spicen up a dead page. Marcis Gasuns is the man.
Download LED Icons here
After noting famfam.com went down some time ago while looking for the great icon set I have decided to help out by providing the entire 1000-icon FamFam Silk set here. Click on the thumbnail below to see the entire set.
Download FamFam Silk 1.3 here
A nice variant on the famfam set is provided to us by pinvoke with the PI Diagona Pack.
Download the PI Diagona Pack here.
For the serious Photoshop folk, Crystal Clear is a fantastic collection of icons that you won't find elsewhere (I used the headphone icon for a recent project and am very satisfied with the result.)
Take a peek at Crystal Clear here.
Tango! is another great resource (albeit an odd one, since it's for desktop environments) that allows you to give a sort of Ubuntu/Linux feel to your website.
You'll want to cut and snip the icons you want with your favorite graphics editor. Visit the Tango! website here