I recently released a new single-player episode for Quake called Deathmatch Dimension. It contains ten maps of classic Quake goodness, inspired by the deathmatch maps that shipped with the original game. Each map has been lovingly rebuilt from the ground up for intense combat with Quake's familiar bestiary, and can be played as a full episode in sequence, or by starting with the shotgun for an extra challenge.
All maps support single-player, coop and deathmatch, and a standard playthrough is probably around two hours long.
Enjoy!