Pitfield Beer Shop, N1 6EY
Regretably this shop closed on 28 Jan 2006. They are hoping to open elsewhere and the brewing is continuing elsewhere. The web site says that they do mail order
Nice ickle shop with loads and loads and loads of different beers. At least four different Krieks, some very nice chocolate beer (Young's Double Chocolate Stout's not a _patch_ on it), Chimay blue, the apple beer that I can't remember the name of, and that's just the Belgian/fruit beer section. It's not particularly cheap, but it's not a rip-off either. The Pitfield Brewery is next door and makes only organic beer.
They also sell home brew kits and equipment for brewing.