Well I came across this site while browsing for icons. All the icons were created by Mark James. The silk icons contains 1000 16-by-16 pixel icons in PNG format. It is licensed under a Creative Commons Attribution 2.5 License. This means you may use it for any purpose, and make any changes you like. All you have to do is give proper credit to mark by displaying a linkback to his website.
Download silk icons
There are two more icon set in his site. you can visit his site to download the other two. :)
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