I made mods for Half-Life and Unreal Tournament. Both are good, however Unreal Tournament was better because the code is much better organized. With Half-Life, I was always jumping from place to place searching through the code to find how to do something. UT uses UnrealScript but it''s very similar to C++/Java and takes no time to get used to.
I actually wrote a couple of Half-Life modding tutorials when I was into it. If you''re interested, there''s one on creating new entities that you can place in a level using Worldcraft and load in your code, and there''s one on creating a Star Wars-type blaster.
Or you can download the code for a mod I started for Unreal Tournament here. There''s a website with screenshots here.
~CGameProgrammer( );