I think this is more of a mix of local/regional and national.
http://www.930.com/
I have never been myself, but I'm sure others have that can chime in.
The 9:30 club is more a concert venue than a club. It's nice and I've seen some great acts play there but it's not the place you want to go if you're just hanging out to admire the scenery and enjoy the local music.
Depending on how you enjoy your evenings, the 18th St Lounge is good. Excellent cigars and fine liquors in a really nice atmosphere.
MCCXXIII is okay if you want a straight up dance club. I used to have an office across the street. If I noticed a lot of girls going in and the odds looked particularly favorable...
I'd consider doing Georgetown during the day and would would skip it for the evenings. There's no metro access so you'll have to take a car or a cab and during the weekends, the main strip is less a road and more a slowly moving parking lot.
Adams Morgan is more accessible and I think it has much more character. There's usually a couple of joints during the weekends with live music. Start with Madam's Organ and then work your way around.
If you're looking for local music, your best bet is to talk the music geeks at record stores in Adams Morgan near 18th and Columbia. The guys behind the counter will point you in the right direction.
Old Town Alexandria's not a bad bet if you're a little older.
One spot to hit up if you're a beer guy from out of town is the Brickskeller, off of DuPont Circle. Over a 1000 varieties of bottled beer and most of it's not skunky, either. It used to be a very chilled, relaxed place before it ended up in a Lonely Planet.
As soon as you arrive, pick up the City Paper.