Zombie Master: Reborn is the remake of the old HL2 multiplayer mod, Zombie Master, where players must fight through hordes of zombies and deadly traps in order to escape, while one chosen player takes the role of the Zombie Master and must stop the survivors by any means.
ZMR is actively under development and maintains most features of the original mod along with re-working features that were missing from the original mod and fixing bugs. You can download the current version, Alpha 7, here: http://www.moddb.com/mods/zombie-master-reborn/downloads.
People normally play on weekends but mostly we'll pop on randomly during the week at around 4 PM EST. The thread usually gets bumped around that time too, so if you're looking to play you should keep an eye out.