Icons (including free for commercial use)
iconfinder.com
iconfinder.com
Examples:
http://www.iconfinder.com/icondetails/7395/128/finder_mac_wi... (XP wallpaper, listed under GPL license)
http://www.iconfinder.com/icondetails/7512/128/application_a... - Apple's icon
http://www.iconfinder.com/icondetails/7043/128/aim_icon
http://www.iconfinder.com/icondetails/7121/128/sonic_icon
http://www.iconfinder.com/icondetails/7175/128/folder_window...
http://www.iconfinder.com/icondetails/7365/128/staroffice_ic...
http://www.iconfinder.com/icondetails/7394/128/users_windows...
http://www.iconfinder.com/icondetails/7563/128/lib_wine_icon
http://www.iconfinder.com/icondetails/7609/128/mp3_sound_ico...
I am also a fan of the Fugue icon set (http://p.yusukekamiyamane.com/) with over 3000 (!) super high quality icons. There is a licensing fee if you don't want to attribute but it is pretty affordable ($60).
There are a lot of differences but there are also a lot of similarities.
I've used the IconSweet2 set for various custom UITabBarItem icons in iOS applications. They fit amazingly well with the System Icons.
What are the legal issues there?
As for why you would want to use someone else's reproductions of these trademarks instead of the official images, I have no idea...
Agreed, Picons are great.
Thanks anyway.