Whats a good easy game to practice modding?

Im looking for a game that i can practice modding for fun. I think i saw something about source modding before but i dont know what would be good for new programmers. So far i have worked some with Bukkit and Modern Warfare 2.
SAMP (san andreas multiplayer) you code servers and stuff. Alot of fun.
C based scripting (pawn) which is not much different, just some stuff is simplified and variables are defined differently.

I don't know of any full c++ ones though.

example code: http://pastebin.com/JK5qq7VW
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