This overly cool set of icons has just made it into my VS2005ImageLibrary folder. There are many icons in the set I've been searching for quite some time to include them in my application toolbars. I made several toolbar icons myself, but they are, well, not as good as the icons in this library. And released under a Creative Commons Attribution 2.5 License, the Silk icons are free to use.
[Found through Cool thing of the day]