OpenPBS - The original PBS from which all have descended. Developed at NASA. Open sourced. Provides simple FIFO scheduler along with custom scheduler to take full benefit of system specific feature. BSD-compatible?
Torque - Resource manager used in combination with Maui/Moab. Fork of OpenPBS after it was abandoned. From wikipedia: "Torque is an opensource job submission software maintained by Adaptive Computing. It is one implementation of PBS (Portable Batch System) derived from OpenPBS. It interacts with a scheduler (usually Maui or Moab) which manages the local scheduling policies, i.e. which job is next to run. All of Calcul Québec's servers use Torque." OpenPBS license
Maui - scheduler. Runs on top of PBS / Torque resource manager to provided better job scheduling. Requires a separate resource manager such as PBS. Open sourced but mostly abandoned. Maintained by Adaptive Computing.