I love street art, even if some people hate it, and don't know the difference between some stupid tags, meaningless graffiti and the beautiful pieces you can see on walls all over Europe. And Barcelona has a really liberal attitude towards this kind of art, so it's a great place to walk around and enjoy the work at no cost!
I mentioned this to one of the guides on the bike tour I did and he told me some great places, but really you just have to walk around and keep your eyes out to see fabulous pieces everywhere. There may be cities where there's more street art than Barcelona, but I haven't found them yet. I've attached a few pics of some favourite pieces; I don't know if they'll still be there, but if they aren't there's probably something new and cool in their place.
Most of the best stuff I saw was in the Born, but that's also where the clean-up crews are hardest at work. It seems like they leave all the best pieces though; I know for a fact that some of the Miss Van stuff I saw in 2005 are still there in 2010.