SMOD is a single player mod of Valve's first-person shooter Half-Life 2. The new content consists of many new NPCs and weapons, new gamemodes, comprehensive customization and tweaking options, new abilities for the player, and new features to the gameplay, such as level randomization and simple branching in the otherwise linear flow of the main episodes.
A definite improvement on HL2. I'm surprised I didn't find it sooner.
My only complaint is that the elites super fast speed is annoying.