There is no way to make any form of a reservation with inopia- perhaps if you are a very large group you could call- but its well well worth any sort of wait, especially if you are looking for something less expensive. Inopia rocked my world.
As for other reccos- I loved bar torres, and a few others that have great energy like inopia- for a full list and photos check out my Barcelona Blog here:
Hi! I went to Inopia last year. It's a lot more casual then we expected. In the "on the road again spain" tv show, it looked like a real sit down restaurant, but it really just is a tapas bar with little bar tables. If you get there early, you won't have to wait long. I think we got there at like 7pm or so, and waited 10 minutes for a top top. Not a real table, they sat us at a little counter.
I have to say to prepare yourself that it might not quite live up to the hype if you saw that show. Michael Stipe and Gwynth Paltrow really made some dramatic moans and stuff. The food was good, I was glad we went, but it wasn't the best food we had in barcelona. It's not that expensive, but we ordered a LOT so it added up. But it certainly wasn't "expensive".
My favorite meal in barcelona was at Pinxtos in La Boqueria, the famous little cafe/tapas/bar in the market. Wow was that food good! Its not a sit down restauarnt at all, you eat at little bar and have to wait for stools to open up. More of a lunch thing. ( dont even know if its open at dinner time maybe not)
Our favorite sit down meal was at Cafe L"Academia (this is from memory, so my spelling wording might be a little off.) That's in the gothic quarter. Amazing food at decent prices and the wine we had for 16 euro a bottle i'm still dreaming of.