The Grim Outpost by Andrew Smith
A large Wizard-/Episode 2-themed fortress that feels like classic Quake goodness.
Disengage the Simulator by mukor
A smallish AD slaughtermap "simulation" in a DE_DUST (from Counter Strike) setting, complete with dead terrorists and plantable bombs.
The Metal Shrine by vaf
Small Metal/runic map with a base part at the beginning. The author's first release for Quake.