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. 

Thanks otp!

lpqsp3:
http://quaketastic.com/files/misc/lpqsp3%20scrap.zip

The map was for AD and was to be called "Azure Anesthetics," a hybrid medieval/base-themed map built out of blue stone, metal, and runic textures. There are some monster/item placements and moving parts, some areas are lit and detailed, others are obviously incomplete, others are not even laid out yet. There are a few places with jumbled bits that I had meant to delete but hadn't got around to yet, and some floating bits I had copied from other scraps and wanted to incorporate somehow. (Yeah, terrible workflow, I know.) I've included all the necessary texturewads (you may have to re-add them manually because I think I had added them according to absolute filepaths); the one called "animated tech" is actually a jumble of miscellaneous textures copied from other wads and individual maps.

Anyone has my permission to do anything they want with the scrap, just credit me. If someone actually finishes it, I will play it, even if I don't play much of anything these days. 

Scraps from an old Rubicon chainmap project where everyone lost interest after few brushes.

rubytron.map & rubytron2.map by Vigil (iirc "2" is the same map converted to regular map format)
scamp_rub2.map by Scampie
4vigil.map by negke
springtime.map by onetruepurple (lol)