If you would like to submit a file, just send a mail to Spirit.
Quaddicted is a Quake website focusing mainly on singleplayer maps (and Quake-themed mods) as well as nostalgia. There are also archives of other things, like multiplayer maps, engines etc. Check out the Archives.
How to install maps or mods? Give the Quake Injector a try.
Spirit is in charge of this website. Try to contact him first if you need someone to talk to.