This is one of the nicest beer bars I've ever been to in any city. The staff are friendly and enthusiastic, and they seem to be ready to talk beer all night long if you show any interest. The bar itself is pretty big and the crowd is lively and diverse, made up of all ages and types. There are three separate beer menus - ordinary beers, Belgian lambics, and a special cuvee menu where everything is listed by vintage. So if you want to compare, say, all the Anchor Christmas beers from 1996-2002, this is the place.
There are two dozen taps and over 500 beers by the bottle - lots of Belgians of course, but also US microbrews, British ales, and probably every Scandinavian microbrew in existence. Lots of whiskeys too, and a good basic food menu. It's located on Sodermalm, just a minute or two from Slussen station.