Whether the topic is Chechen rebels, Al Qaeda, the connections between the two groups, or completely unrelated terrorist organizations, the Council on Foreign Relations has put together a very informative web site called Terrorism: Questions and Answers. I highly recommend it for information about terrorist groups, failed states, weapons of mass destruction, and homeland security.
The Terrorism Research Institute has both the "terrorism dot com" and the "terrorism dot org" addresses on the web and is well worth visiting, although much of the information is proprietary and requires a subscription.
If you have recommendations for other sites on terrorism, please leave a comment.