Anandtech is kinda big. Around 100K registered users. They've written a couple articles about their forum and web servers. Pretty interesting. I gotta say, I can only remember their forums going down once since the move from ubb a few years ago.

There used to be a forum called Delphi that was huge. It was more of a meta forum though. Their interface was so unbelieveable bad though. I guess the discussion groups on lycos and yahoo, etc. Don't count either.