Red Onion. It's on the northeast corner of 18th & T streets. The entrance is on 18th, right next to the dry cleaners. The selection is decent, but pretty small right now. The owner -- an amiable fellow -- said that it's just a small portion of his stock and he just hasn't had time to get everything out yet. Prices are good, but don't neglect to inspect the vinyl - some of it is pretty scuffed.
I saw Ian Svenonius in there today, browsing through the Soul & Funk section.