I was considering making this post a series with a bit more detail about each venue, but reckon it's probably more useful all mixed into one. Feel free to chip in with the comments if you feel I've missed anything worth a mention.
Barcelona is a great place for live music. As well as smaller venues across the city playing live music every weekend, and endless musical festivals from Jazz to electronic, if you want to get your rocks off in Barcelona, there are plenty of places.
Let's start with the big guns - Palau Sant Jordi. Part of the 1992 Olympic Games, Palau is a very versatile arena, and is the place most of the big artists choose when touring here, as it has a capacity of 20,000. Elton John recently finished his Red Piano tour here and artists as diverse as Green Day, Red Hot Chili pappers, Beyonce, Madonna, Phil Collins and and George Michael (who started his 25 Live Tour here) have had no problem filling the seats. Palau Sant Jordi is located up on Montjüic, meaning there's plenty of space for revellers to queue up before hand and public transport links are excellent.
Another popular place for the alternative crowd is Razzmatazz. This is actually a huge club with various different areas catering for all people's tastes and is located over in Poble Nou. This is a great venue to see bands who have not quite stamped their mark on the Spanish pop map just yet, and often bands play here before returning to the larger venues once their fame has took off. I've seen some great bands here; Coldplay in 2003 who later returned the same year to fill Palau Sant Jordi, Travis, Keane, NOFX, to name but a few. An excellent and intimate venue which is here to stay.
One of those intermediate venues is the Pavello Olympic in Badalona. This is an ideal venue for those bands who have a fairly large following, but can't quite fill Palau Sant Jordi (sting played a couple of years ago and should have gone here!). with a capacity of 12,500, the likes of Take That and the Backstreet Boys are happy to get bums on seats here and it's a decent size sports arena with great acoustics.
FC Barcelona's Camp Nou is reserved for only special guests, and obviously plays off season. The most notable acts over previous years have been Irish rockers U2, who played here in 2006 and started their recent 360º World Tour here with a double bill on 30th June and 2nd July this year. Camp Nou is an amazing concert venue, saved for über-stars, and was packed to the rafters on both occasions (I was lucky enough to go on both tours). The recent Go Crazy video was shot in Camp Nou (although they played the more upbeat version!).
Sala Bikini is a really small venue up near Diagonal, and again, is a great place to see some famous bands who have not necessarily made the grade over here yet. One of the best concerts I saw here was years ago when Supergrass played here - such a small venue and there was no barriers between the band and the crowd, it was amazing. Another place which I haven't seen many return, but is still a decent sized venue is Pavello de la Valle d'Hebron, in the north east of the city. Oasis famously cancelled a gig here in 2000 which was the slippery slope to where they are now, as they then returned without Noel and played the fastest concert I've seen (all the songs played at twice the speed up to finish earlier). When they finished, there was no encore, and Liam Gallagher said "Right we've been here now, f#*k off" - as if it was a simple contractual agreement! Never mind!
There are then some equally cool but more sporadic venues around town, where live music can almost always be found. This is where I'll need your help in the comments guys! I'm talking about the classic places like London Bar, Harlem Jazz Club, Jamboree, Pipas Bar, etc. These are institutions in Barcelona, and many a night we've stumbled across a live band jamming away and spent a great, un-planned night in one of these places.
Coming to see a band in Barcelona soon? Check out these apartments for rent in Barcelona and for a heads up on the rest of the city, see this great Barcelona Guide.