Interesting question, especially the "why" part.
The High Road - most diverse group of folks I've found (except TFL) and usually can find considered opinions of the widest variety of subjects. Also, good for at least one belly-laugh/day.
1911Forum - my favorite pistol. I'm hopelessly addicted and this forum has a great core of folks with more knowledge than I can hope to assimilate. Good reading and good advice.
Ruger MarkII & 22-45 Forum - went here initially for specific questions about my MarkII. I go back once in awhile to ask other questions. Too many pop-up windows to deal with, though. The format is also not to my liking. But, a fairly good resource.
Marlin Talk - have been considering purchasing a Marlin. Good place for information.
Rimfirecentral.com - if you own a .22, you need to visit this forum. Excellent resource and fun bunch of guys.
HS2000 & SA-XD Talk - have been considering an XD-9. These guys are fanatical about this gun. Very informative.
CZForum - similar passion about the CZ guns. This might be the most civil, friendly forum I've found. And, that's no slam on anyone else. Just to say these guys are very helpful, very enthusiastic about their choice in guns and go out of their way to make newcomers welcome.
Pistolsmith.com - just started visiting (lurking) here. Lots of wonderful technical expertise. If you have a technical question, ask it here.
The Firing Range - of course, this started up right before the demise of TFL and it is a comfortable place to visit. Not a real large group of folks, but a come-as-you-are, take your shoes off and stay a spell type of place.
Tactical Forums - a more over-the-top hard core bunch of guys. Interesting and these guys are serious!
Packing.org - as already mentioned, THE place to go for up to date CCW laws. Usually some good topics to post to also.
Don't forget Yahoo Groups. I belong to 2:
IDPA - just what the name indicates. But, in addition to discussions of competition related topics, a lot of interesting general gun-related stuff, too. I like these guys (and gals) and since I am looking to get involved in the sport, it's fun to lurk here.
Concealed Carry - Probably the most interesting group of folks I correspond with. Every now and then they even talk about CCW! Mostly lots of sit-around-the-wood-stove discussions that are just plain fun to be a part of.
Seems like whenever I get ready to investigate a new firearm I find a forum and visit for awhile. Works for me.