czg's scraps:
http://www.quaketerminus.com/hosted/happymaps/maps.htm
Lots of scraps, ranging in quality and size. All of them probably very old, but still usable (as evidenced by how often they're recycled in newer maps).
Bal's Nehahra maps:
http://www.born-robotic.net/content/maps/balostepisode.zip
Made for a Nehahra episode that never materialised. Huge and old. Posting them here only for historic value since he's been making new stuff based on them instead.
RemakeQuake scraps:
http://svn.icculus.org/remakequake/
Lots and lots of stuff, maps, models, sounds, code etc. All (presumably, never got a proper license) ripe for grabs. There's some real gems to be found if you dig deep enough.