Half Life 2 Roleplay (HL2RP for short), is a serious roleplaying gamemode for Garry's Mod that is set in the universe of Half Life 2. There are a few different versions of HL2RP, but the most popular is Clockwork. Unlike most gamemodes, HL2RP appeals to a specific crowd of serious roleplayers.
HL2RP is so extensive that it has it's own wiki. It can be found here.