I spent some time (a lot of time, actually) putting together just about all resources regarding maps together into one place.
http://www.quakeone.com/maps
The top of the page is more geared towards server map packages (Rocket Arena, RQuake, etc.)
But after that top section, are links and banners on everything about maps from Quaddicted's archive, to map making resources, level secrets, single player map reviews, to Quakeworld tricking links, etc.
(yeah, this only has screenshots of, I think, 160 to 170 maps ... way short of the grand total # of DM maps)
If you notice an omission or something useful that could added, let me know and I'll do that.