I loved the mods for the holy trifecta: Quake 3, Unreal Tournament, and Half Life. I consider it the golden age of online gaming. I also spent a lot of time on IRC servers chatting with developers (eventually joining a team myself which lead me to my current career as a developer). My favorite was for Weapons Factory, which was a popular Quake 2 mod at the time. The old dev team moved on with their lives so the community took over...and it was actually released and was even better than the first. High quality, with tons of cool maps, and for an active community of maybe 1000 people? Insane.