Forums: I spend a bunch of time reading and posting to several forums. Here's the list:
TimeForTuckerman A group of people who ski Tuckerman Ravine. I'm crazy active on this one, reading almost every post, and currently hold the "High Poster" position. I've met a few dozen people on this board and regularaly ski with them in the ski season, and see them a lot during the spring skiing season.

Merrimack, NH Forum A forum focused on the town I live in. This forum is often an example of how NOT to post to a forum. I'm overwhelemd by the situation where people post a longwinded, non spell checked, opinion filled rant with questions in it like, "Who's job is this?" Got me thinking of tracking how many questiosn go unanswered, mostly becuase there is no answer.

NoHomers Club A simpsons forum I found recently. Wicked high volume - I don't read more than 1% of the posts - but it's fun to check from time to time.

SugarbushRSN and SugarbushMRV
These both revolve around the ski area Sugarbush. The RSN one is very old (like 4 yrs, old in internet time that is) and has weak forum functionality, recently there is talk of it going away. The new one is an attempt at getting things into a more modern format, and also in anticipation of the RSN forum disappearing.

SonsOfSamHorn forum: This one of the best red sox forums, but I don't post here, I'm no wheres near as knowldgeable on the sox as these guys.

Section39.Com This is the board for the people who sit in the bleachers at yankee stadium. I'm a die hard redsox fan, but these guys are masters in the art of flagrant and rude insults. It's been fun rubbing in the Red Sox world series win.

There's a few more I have participated in, but these are the big ones.