Street performers are one Barcelona's major attractions. Most of them are located in the Ramblas also you will also find them in the Gothic Quarter, specially around the Cathedral.
Among the street performers, I will highlight the "human statues" as some of them are impressive although my favourites are the singers. Some of them even sell their own CDs, so if you like their music, why not spending a few euro on a CD.
It is also not unusual to see tenors and sopranos, and in general quality of the street singers is pretty good, so if you have the time, why not spending 5 minutes listening to them as you may be impressed and you may end up standing there for a good while. You can even grab a coffee and come back to listen to them, I am sure they will like, specially if you show some financial support.