For anyone planning to be in Barcelona from the 24th-27th there's a huge festival going on.
It involves all kinds of things from free music concerts all around the city, to crazy fire parades, to wine festivals, to human towers, fireworks, and a whole lot more. It's basically all day and all night.
The complete program can be found here, but it's all in Spanish
http://www.bcn.es/merce/ca/programa-actual.shtml
An English version of the highlights can be seen here:
http://www.siabarcelona.com/MerceEng2009.htm
Oh and all museums in Barcelona are free on the 24th.
cM