This set of 160 icons is provided by The Working Group, and are licensed under a Creative Commons Attribution-Share Alike license. You may use them for both commercial and non-profit purposes, as well as modify them as you wish. Each icon is available at 3 sizes: 64 x 64, 48 x 48, and 24 x 24.
Download here (please inform us if the link is broken! We keep a spare set.)