Nehahra
This is an awesome single-player mod for the original Quake. It even
includes a movie, using the game engine to act it out. Nehahra
is one of the best Quake mods I've ever played.
QuakeForge
This is one of the most popular source ports for the original Quake.
QuakeForge improves the network
code, adds major benefits to the rendering engine, and increases performance.
Developers are still working on the port to improve its functionality
and appearance.
MegaTF
This is a multiplayer mod based upon the immensely popular Team
Fortress Classic. MegaTF
improves upon the original by adding such features as "prone"
mode, improved weapons, and measures to reduce lag in multiplayer games.
It looks like the creator has halted all work on the mod, as the last
update to the site was nearly two years ago.
Rail Arena
This is a modification for Quake III. It transforms gameplay into a sniping
fest, using only railguns, and one shot kills your opponent. Rail
Arena really quickens the pace of gameplay and increases the fun factor.