I spend alot of time at

http://www.skyscrapercity.com

which is primarily, as the URL suggests, a site about skyscrapers around the world, but the section i frequent relates to architecture and construction within my home city, Manchester UK. I'm a fan of it all and there are some knowledgable people in there, so if you are into it, I reccomend it.

http://bbs.stardestroyer.net

is another place i waste my life away at, it's a star wars vs star trek forum (attached to a great and extensive site). I like it, can argue about trivial and unreal things as well as have some great discussion with the 60 or so members online.