Genericons is a set of very clean, vector-based icons embedded in a webfont, which can be scaled to any resolution, or even styled with CSS.
The upcoming theme, Twenty Thirteen, which will be included in WordPress 3.6 makes use of Genericons, so check out the demo of that theme to see it in action.
To make it even easier to begin using Genericons, Mika Epstein has already released a plugin called Genericon’d. Install the plugin, and then begin using either HTML or simple shortcodes to display Genericons in your posts, pages, and widgets.
If you’re looking for a free set of icons to use in your site or other project, look no further than Genericons.