The folks over at Smashing Magazine have composed a set of 750 free icons with their .psd sources. The original post is a bit old but this is still a great resource for web designers and developers in the need of thumbnails. LINK
[Smashing Magazine]
The set consists of web application icons, user interface icons, e-commerce icons, general use icons, desktop icons, blog icons, smiley icons, button icons, keyboard icons, national flags icons, popular online services icons and many others.
[Smashing Magazine]
