Stanford Wong's
www.bj21.com has already been recommended. It's nearly impossible not to get a good answer to any BJ question there. Luminaries: MathProf, DD', Cacarulo, Don Schlesinger, Norm Wattenberger, ET Fan, and many others.
Arnold Snyder's
http://blackjackforumonline.com is primed towards the professional player. Lots of active, full time pros freequent the pages -- but can be understandably stingy with information. The articles and the archives of the site are full of exquisite stuff. And Snyder's Blackjack Forum is now an online magazine. (The site also sports the internet's only Fight Club for BJ aficionados, a decidely stupid affair!)
Viktor Nacht's
http://www.advantageplayer.com is a serious BJ website, where experts roam -- the names mentioned above, for Wong's site, are here on a regular basis, particularly Don Schlesinger.
Richard Reid's
http://www.bjmath.com is full of forums, and aims for the serious, math-inclined student of the game. The feedback one gets in the free area is usually of the highest class. Highly recommended.
Michael Dalton's
http://www.bjrnet.com/board_posts.htm is rather sleepy, but if you wanna get some info from T-Hopper, possibly the strongest card counter today, or catch a glimpse of Gehrig, an obscure and rather cult figure of Blackjack, this is where you wanna put a question about the game -- or Las Vegas. Go on, ask about the old days!
There are also some other Blackjack websites, rather specialist in nature, or more casual (such as
card counters' cafe or
The Mayor's), most of which you will find in the Links pages of the sites named above.