BlendMeIn is a nifty new Photoshop and Illustrator extension that allows you to search thousands of assets, including popular icon packs, without leaving Photoshop or Illustrator, and place them in your document directly via a Panel.
Unlike FlatIcons, which I recently reviewed, the artwork available in BlendMeIn is free via Creative Commons Attribution license. Unlike FlatIcons, it works in Adobe Illustrator as well as Photoshop. I still prefer FlatIcons, but this is a great option.