L'Institut du Monde Arabe in Paris is an astounding building. Covering the entire south side of the building is a wall of adjustable irises (detail to right, more photos here) designed to let varying amounts of light in depending on available sunlight. Inside the building, everything is exposed...polished stainless steel (or aluminum?) beams & poles everywhere. I wanted to spend all day in there, poking around, but there were other things to see as well.
More photos (not mine): photos of London, mostly public signage.