De Halve Maan ("Half Moon") in Brugges. They make the decent "Brugges Zot" beers, but the brewery tour is REALLY good.
You get to go through the entire brewery up to the roof top (by the old open fermenters), there is lots of old Belgian brewing equipment everywhere throughout the tour including the old HUGE copper wort chillers. -They'd run water through the inside and then run the beer over the outside and let it cascade down (definitely not the most sanitary thing ever; probably one of the most non-sanitary things ever). They even used to have their own malting floor which is part of the tour. Really cool crawling around in little tight areas.
Delirum Cafe is a must stop in Brussels as they hold or held the world record for most bottles of beer available for purchase...