The Warehouse Nextdoor is a nice little place to play or see a show in DC. It's between the Chinatown and Convention Center Metro stops and is run by a small dedicated group of people who want to provide a place for bands to play that can't get into the bigger clubs.
1019 7th Street NW
202 783 3933
Shows usually range $5 - $9 and are all ages.
http://www.warehousenextdoor.com is now the most up to date listing of shows.
The Warehouse recently moved into the space next door....which actually makes it the warehouse next door, since the warehouse is actually the cafe, and the warehouse next door was actually the warehouse next door to the next door....