I ate at Bar Mut at lunchtime in June, 2008 and LOVED it. This was my favorite tapas experience while in Barcelona. Good mix of locals and tourists. The waiter made excellent suggestions (some sort of tuna with a guacomole type base; and a seafood plate). Ask for suggestions here and you will be rewarded!
I also really loved going to the Mercat de la Boqueteria for breakfast. Try Cafe Pinotxco and El Quim. Both were probably two of the best eating experiences I had in Barcelona. Inexpensive and delicious-plus, the people watching is fantastic.