ProjectCarthage (also known as PRC) is the core plugin of MRCL. It contains every basic function of the server.
It is not to be confused with the show's Project Carthage, for which Franz Hopper worked for.
Most of MRCL's functions are inside this plugin, here's a non-exhaustive list of ProjectCarthage's systems:
However, ProjectCarthage is not the only custom plugin coded for MRCL. For example, most XANA attacks are actually added by other plugins, as well as Sector Five's maze.
This plugin is still the oldest and most important one and the server would not be able to function without it.