The Noun Project

The Noun Project is one of the very best places to get icons depicting just about anything. There are a huge number of icons available, and all provided as vectors so they can be scaled as large as you wish.
As long as you attribute the designer, you can download and use the icons for free. Alternatively you can purchase a royalty free licence at a very reasonable price (currently $1.99 for a single icon). You can also take out a NounPro subscription package that affords you unlimited royalty free icons.
The Noun Project recently introduced a Mac application for NounPro subscribers, this is a big time saver as you can drag and drop Noun Project icons directly into your favourite graphics applications.