Welcome to the less rockin' town in Spain!
Definitely, nightlife isn't very exciting over here but not all is lost: Dulcinea is right, Barbara Ann is the place to go, even if it's away from the city center. More a bar than a club, but always with great music, ranging from 60's Beat to today's Punk Rock, through Garage, Power Pop and Soul. The same can be said about Las Guindas (C/ Sant Pau, near Ronda St. Pau) and Big Bang (C/ Botella). Malpaso is OK too, but it gets too crowded easily. Dr. Alga is wrong: It's not on Plaza España, but in a backstreet behind Plaza Real. Las Guindas and Big Bang are kind of close, and not really that far from Malpaso. You can walk (but be aware that some points in that area can be not really dangerous but you get the idea). And there's also the Apolo Club in C/Nou de la Rambla, very close to Las Guindas, with two separate venues that usually have the best concerts when there's any.
The Sidecar club is also OK, but only if there's a good gig. Also in Plaza Real, very near Malpaso.
I don't really go out a lot lately, but I don't think there's much else.
If you'd like to eat some good tapas in a good atmosphere (rockin' people mixed with assorted youths and old fellows from the neighborhood) try Bar Ramon, in the corner of C/ Borrell with C/ Tamarit. Nothing really sophisticated, but you find good food, cool people and old R&B/Jazz/Rockabilly/Soul as soundtrack. (Friday night is usually packed -try some other day).
http://www.myspace.com/barramon
By the way, it's just across the Sant Antoni market (on Sunday mornings: mainly good for books, old magazines, some records, etc.
The rest of Dr. Alga's recommendations are good: If you're looking for records, C/ Tallers is the place. The best stores are Overstocks and Revolver (specially the one with vinyl upstairs). Around the corner there are also Daily (as PJ says) and Kebra (there are two, the good one's at the same side of the street as Daily, with lots of vinyl -Good luck in finding this one open, as the owner has a "timetable" of his own!), both in C/ De les Sitges, just around the corner from both Revolvers. C/ Riera Baixa is well worth visiting also, with Wah Wah and Edison's, two cool record stores that are nearly door to door.
Penniman is no doubt the coolest label and mailorder in Barcelona, but they don't have a store. The same thing goes for Butterfly Records:
http://www.butterfly-records.com/
If you're into 50's and Rockabilly there's this festival just 1 hr. by train:
http://www.50srockersclan.com/home.htm
Some other really cool bands from Barcelona:
http://www.myspace.com/thefabulousottomans
http://www.myspace.com/thecanarysect
http://www.myspace.com/biscuitrocks
http://www.myspace.com/bornlosersbcn
Keep in mind the following:
-September is still very hot in Barcelona.
-Most signs are written in catalan, not in spanish, but they are pretty similar.
-(I hate to say this) Beware of pickpockets! Everywhere!
-September 11 is holiday in Catalonia, so stores are closed. Good day to enjoy the city's architecture and historic places or to visit museums and stuff like that, just make sure they're open that day. If you're in the center, chances are you find different political demonstrations, both official and "protesting". Don't be afraid, but try to stay away from (unlikely) trouble.
-There's an open air flea market on Plaza de las Glorias on Mondays, Wednesdays, Fridays and Saturdays. If you're serious about buying interesting stuff, forget it, but miracles do happen sometimes, I tell you. Anyway you can always have a good time looking wacky stuff and people. Good for books but you have to be