As my wife would undoubtedly agree, I waste a lot of time online when I'm supposed to be doing my schoolwork.
I'm very active (compulsively active?) on a skateboarding/skateboard collecting forum--it's mainly children of the '70s and '80s (though we've got plenty of guys who grew up in the '60s, and a few from the '50s and '90s) reliving their salad days by purchasing their childhood one deck at a time and by talking and skating with other "older" skateboarders.
I'm on another skateboarding/longboarding forum mainly just to coordinate our weekly sessions. The rest of the site is overrun with pubescent know-it-alls and young college kids (with plenty of blind sycophants thrown in for good measure) who think they know everything because they've been skating for a whole year or two.
I lurk a few Fighting Illini sports boards, but there's a weird schism between boards regarding support for the coach (I can't believe any, let alone how many, want him gone), focus on recruiting, and a whole host of other things. The last couple of seasons really hurt the quality of the message boards. Many people expect that we should be in the National Championship game every year and that anything less than that is unacceptable. I mainly just lurk for news and tidbits with an occasional irresistible snide comment thrown in here and there. Living 1500 miles away from the campus, the interweb keeps me abreast of the news, but honestly, it also makes me a little ashamed to be an Illinois fan. A small but vocal minority of our fanbase are real turds.
I post on the Kansas sports off-topic board. I no longer harbor any particular ill will at KU for stealing Coach Self, and the Illinois off-topic boards either have obnoxious interfaces, obnoxious fans, or are almost entirely political in nature.
I occasionally lurk or abuse the search function at NASIOC and legacygt.com but I really don't post. I don't know a whole lot about the mechanical aspect of cars (just how to drive them) and the one time I posted at NASIOC looking for guidance I got chastized and flamed, just as I expected I would. Many of those guys seem like real dicks.
I've been lurking and abusing the crap out of the search function at radardetector.net and other assorted stealth-related sites lately as well. I'm not a radar/laser hobbyist or enthusiast, but I do enjoy driving fast so I try to keep abreast of all the stuff I need to know. I just got a V1 (anyone want to buy my Bel 995???) and am likely going to be installing a Laser Interceptor system soon as long as I can make a decent amount of money on a few skateboard decks I've got up on ebay right now.
I also occasionally lurk or abuse the search function at a forum for greyhound owners--particularly those with retired racers. They're very unique and occasionally high-maintenance pets (think of owning a Ferrari with 30,000 miles on it...) and the people are kind but a little batty (there is no shortage of posts deliberately written as if the dog had somehow learned to type phonetically, for example) so I mainly just go there when we've got questions or concerns about our dogs. It is fun to look at all the pictures of the members' dogs though.
I am not active on any other cigar forums.
I think that's about it.