Abandoned Places is the old standby for the rotting industrial sites of Belgium:
http://www.abandoned-places.com/
Modern Ruins expands on those themes to include sanitariums, prisons, and boatyards in and around new york state and pennsylvania:
http://oboylephoto.com/ruins/index.htm
This guy has tons of travel photos featuring architecture from around Europe and the world:
http://www.photo.net/travel/
Buffalo Architecture and History has features on hundreds of buildings located in Buffalo, NY, and information on major architectural movements embodied by those buildings:
http://ah.bfn.org/