544 vector symbols and counting, representing the visual language of the world, is what you’ll find at the NounProject. Clicking on an individual icon will reveal what the icon means (when known) and allow you to download it. All superbly created vector icons are free for personal and commercial use. The only downside to the site is there is no way to download a single archive of all the icons.
If you’re designing apps or comps for the iPhone 4 with a retina display (640×960), you’ll find this collection of iPhone 4 GUI elements quite helpful. This fully layered Photoshop file contains all the elements you need, and weighs-in at a whopping 4074×2986 pixels and 63MB in size.