18th Street Lounge is definitely my favorite lounge in DC. It's owned by Thievery Corporation, so the music spun is mainly downtempo with a bossa flair (think Nicola Conte, Baden Powell), and a live band playing the third floor on weekends. Some people dance, but most sprawl and chat on the numerous couches.
Dress Code: It varies. The best advice I can give is to aim for trendy, a la New York City's club scene. You'll see people sporting faded jeans, Armani suits, and nearly everything inbetween. The only sure-fire way NOT to get in, however, is by wearing khakis.
One of the very popular club. Live bands sometimes play there too.
But beware, gentlemen, sometimes they refuse to let men-only-group to enter because 'there aren't enough ladies' in the club. :) My friends were banned from entering one night. So, get a lady to go with you!
Also, the coverage is $10 I think, not sure though.