Before Shmabler's site there was Matt Sefton's site
and before that there was pure crap.
I remember sitting at my school downloading maps from cdrom.com onto a 1.4" floppy and bringing home, and they were unanimously terrible. It wasn't really until late 97 or so that people started making decent maps.
In particular I remember the horror that was the Aftershock pack (which led me into mapping, so yay!) which came packaged with ~60 or so custom levels. One was a giant box with the pink/black checkerboard textures (which renders as HOM in GLQuake). Another was a box that spanned the entire grid with lava at the bottom and thin walkways across it and lots of shamblers. Other highlighs included ghosts.bsp, which was pitch black, filled with monsters and slime, and no exit, and swamp.bsp which was a lovely creation of vertical water brushes and flying fish.
Another thing you might (not) want to check out is After The Fall
a total conversion which is pure shock and awe because oh my god it's so terrible I'm crying just by thinking of it. Sound effects that sound like a lewd phonecall, textures made from photos of motherboards, the completely nonsensical bolo gun... BIKER BITCH NOO!
So what I'm saying is that the only thing that is worth checking out really is the stuff listed on Matt's and Shagbler's site, and you can find plenty of original stuff there too.