If you want cheap and crowded, this is the area. Reef has a good rooftop bar, Madam's Organ gets packed and loud, Kokopooli's is a basement bar, dark and smelly, but not usually too crowded (haven't been there in a couple of months). Basically, you can just walk along 18th Street there and wander in any place. A real dive is Dan's. They sell liquor by the small bottle (not airplane bottles, more like flask size). You buy the bottle, get a couple of mixers, and a bucket of ice. Needless to say, you can get pretty banged up in there.
Millie and Al's is a pretty cheap place, as well. A much younger crowd, jello shots, great fun atmosphere, loud and crowded.

For sheer concentration of bars, Adam's Morgan is the right area.