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
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.
What are the legal issues there?
Agreed, Picons are great.
Thanks anyway.
I've used the IconSweet2 set for various custom UITabBarItem icons in iOS applications. They fit amazingly well with the System Icons.