SourceMod (server plugin)

Not to be confused with sourcemods (the Steam folder).

SourceMod is a server modification that gives server admins a much more powerful platform for scripting and administration. While mostly used for multiplayer games, it can be installed on any Source engine game. Many well-known server plugins are powered by it, such as Roll The Dice (RTD).

It has a lower-level system that needs to run alongside it, called MetaMod.