I’ve spent many a lunch hour here on SDMB, mostly lurking in GD, and I have been impressed by the level of discussion there. From time to time some posters refer to other message boards but do not name them. I have learned a lot here, but I’m ready to explore discussions in some of the other venues. If it’s not against the rules, I’d appreciate suggestions for other high quality message boards. Not that I would expect to see anything that surpasses what I see here; I’m just interested to see what else is out there.
Wow, don’t get out on the internet much, do ya? 
While I can’t speak to quality, Big Boards has a list of the larger, more popular message boards and you can search by topic, rating, or other statistics. In terms of size, SDMB currently ranks 96th on their list with only 6,714,452 total posts.
Size isn’t everything!
Based on my experience, the SDMB is far more knowledgeable and sensible than your typical Internet Message Board.
(I c u r wondering how I no - well lack of textspeak is one good sign!)
Agree with glee- try going to the IMDB boards sometimes and try and find a thread with more than 10 posts that doesn’t dissolve into “fuck u”, “no fuck u” at some point.
DSLreports.com is pretty good. Not all the forums mind you, but the one’s I frequent. (Digital Imaging, some of the hardware forums etc…).
You might try Opal’s board, “The Fabulious Fourms of Fathom” aka http://fff.fathom.org/forums/index.php?s=
(I dunno how to do the linky-link stuff) It’s a good board and some of us go there for a change of pace and some damn friendly people!
This isn’t going to add much to your variety, but I frequent the JREF board under a different alias:
I hang out over at www.tivocommunity.com (as tanstaafl there too) and I know a few other Dopers do as well. They have fairly large IMHO/MPSIMS and Cafe Society equivalents. They actually discuss TiVos too. Sometimes.
I used to love that forum, but a few years ago it seemed to degrade into a bunch of whiners and complainers. For example if I ever hit the TiVo button and saw a yellow star I knew we’d get a bunch of new members signing on to bitch about how much they pay and how they should’t be subjected to advertising blah blah blah. I still stop by if I’m in the mood to help someone troubleshoot something or if I have a question. And I still send people over there when they have questions I can’t answer about their TiVo, but I don’t hang out there like I used to.
Snopes has its moments, but those dreadful and abusive pop-up ads!
SPOOFE and I are members here:
Moved to IMHO.
-xash
General Questions Moderator
I also frequent (under the same user name) the boards at fuckedgaijin.com
It’s not as big or intellectually stimulating as the Dope, but it’s closer to home.
Yeah - not what it used to be either. Which I guess is linked to the main site, which seems to be obsessed with partisan politics now where it never used to be…
I visit the www.twoplustwo.com forums frequintly. You know so I can atleast get a tiny bit of poker knowledge.
I personally like both jeepsunlimited.com’s forums and the pirate 4x4 boards. It’s where you learn the 4 letter word to shut people up: G-H-E-Y. Ghey!
Good board; rather like a giant SDMB BBQ Pit in tone however. I don’t mind, but some do.
I also visit the Baen Bar ; good stuff, if understandably leaning towards discussions about fiction.