Entypo Icon Set
Entypo is a set of 100+ carefully crafted pictograms available as an OpenType font, vector EPS and web font. All released for free under the Creative Commons license CC BY-SA.
There is not much more to add. Entypo covers many common icon use cases and the fact that it ships with an OpenType font makes it even smoother to use. I use a few icons here and there on this site and mainly prefer this icon set for its cleanness and simplicity which allows you to easily integrate it into most designs.
Below you can see some of my favorites from the set. The cool thing about having it as a font is also that you can change colors very easy. Deprecated the demo.