Here at Quaddicted.com you can browse and download all known and preserved Quake Singleplayer Maps. Probably there are still a few missing ones, please mail anything interesting to Spirit.
There are some more projects by Quaddicted.com/Spirit as well as some mapper's homepages:
Credit for the greatest amount of the descriptions and sorting goes to neg!ke, OmegaPhlare, Sielwolf, erc, Megazoid, ijed, MadFox, spd in no particular order (I have that feeling that I forgot someone, please tell me if!). Read this if you want to write descriptions too. Thanks for help on converting all the data and various html/css things to mwh, aguirRe, Lardarse, Trinca and the #tf crowd. The Quaddicted logo was made by Husker. The icons (Tick and Cross) are from the nice Silk set.
German internet laws are fairly weird. I better have my address here. You can use it to send chocolate cookies and love poems to me.