Just discovered Game-icons.net and it is so wonderful.
Search for icons by category on the right (Symbol, Action, Machine, etc).
Icons are provided for use under the Creative Commons 3.0 BY License.
If your browser can handle it you can see a full preview of most (all?) of their icons in one sheet here: http://game-icons.net/images/montages/game-icons.net.png
Be warned, it takes a while to load o_O