Which websites have active bitcoin communities with discussion that is going on?
Bitcoin Talk is currently the most active.
The bitcoin reddit also has a fair amount of traffic and discussion.
After that, it falls off pretty significantly.
There are of course often discussions about blog posts or news items on tech related blogs like Slashdot, Ars Technica, Wired, etc. The same is true of some sites dedicated to online gambling, and some Tor forums dedicated to the Silk Road and similar projects.
There have also been some longer threads that are especially useful for miners at the Overclockers Forum
Witcoin. They have an interesting concept to keep spam down: You have to pay to post (using Bitcoins). Revenue gets shared between the site and the poster your replied to.