Other games?

Are there any other games (modern ones if possible) that are as easily moddable as Quake 1? Just wanted to try smth new...

Anything beginning with the name "Unreal". UnrealScript is limited but it's very much like QuakeC in the concept of being crossplatform game logic code that doesn't depend on some big proprietary compiler, but infact it's a included tool

Hmm, unreal surely is a good idea, thanks leileilol. Just one more question for the topic: has any1 tried GTA 4 scripting? Is it worth spending time on?