One of the more interesting libraries I’ve used recently is Font Awesome. It’s a glyph library that allows you to use professional clean looking icons as a part of your project. It is very flexible and is easy to implement using markup. It is part of the Twitter Bootstrap world, is easily integrated into your website, and is also able to be added to WordPress via a plugin.
One of the nicer features of the library is the simple built in functionality to do things like make the icons larger, stack them, flip or rotate them, and much more.