This somewhat misses the point. The reason why FamFamFam (aka Silk icon set) is so popular is not just because it's free. There are loads of free icon sets out there. However, Silk is consistent and extensive. It has a consistent graphic style throughout, and covers an extensive range of possible icons.

Using icons from half a dozen different icon sets will make your application look a bit Frankenstein-like. All those other icon sets are way too small to be of any use in any full-scale web app.

there are other sets like credit card icons or country flags that are fairly independent.

thanks for the link. here are 50 more: http://speckyboy.com/2009/02/02/50-of-the-best-ever-web-deve...

Icon sets. The Kai's Power Tools of web application design.