Unity is a huge collection of stock icons. 225 in total!
The package includes a variety of file formats including: PNG, ICNS and ICO. Something for all platforms including web design, iPhone App development as well as Macintosh and Windows application development. And frankly whatever else you can think of.
Each of the 225 icons comes in 3 different sizes:
48 x 48 px
32 x 32 px
16 x 16 px