Remake Quake (RMQ) is a Quake mod. It requires the full, registered version of "Quake" by id Software to play.
RMQ contains mainly singleplayer maps, but also supports multiplayer.
It is in development since 2007.
The team has released several demos already; interested players can download them from the RMQ development blog. Just click on "Downloads" there. The latest singleplayer demo comes with an EZ Installer for Microsoft Windows.
Subversion web frontend
Anonymous Subversion checkout (for developers):
svn co svn://svn.icculus.org/remakequake