Flickr HDR Group: There is some good information in this discussion group. The only problem is you have to sort through a lot of what I call idiot posts. People need to learn to read threads before they post a very basic question. Overall though, there are 18,000+ members of this group and they produce a lot of very good HDR images and discussions. Like any message board, there are some very good photographers that post regularly. I like to read through this group every couple of days or so.
Stuck In Customs HDR Tutorial: Trey Ratcliff's images are amazing. The link is to his HDR tutorial. Anyone interested in HDR imaging should read this site thoroughly and appreciate his impressive images.
Cambridge in Colour: This is one of the best photography sites I've read in a long time. Sean T. McHugh's website should probably be categorized under General Photography, but most of the work on his sight is Night Photography related. But, his tutorials and techniques section is by far some of the best information I've seen on the web for digital photographers. The tutorials delve into the mathematics and inner workings of how a digital camera functions. I hate math and it gave me a headache trying to learn to understand it, but it was a headache well worth having.
Lost America: Troy Paiva is an exception night photographer. His works center around ghost towns and abandoned structures in the southwest United States. There's not a lot of information on his techniques, but the images are great. It's worth an occasional look to see his new photographs.
The Nocturnes: This is kind of a catch all site for night photography. There are very informative articles and photographs published frequently. I don't reference this sight as much as I once did. The photographers that maintain the website are based in San Francisco and hold yearly workshops.
Sports Shooter: I did sports photography for so long that I'm trying to get away from it. Even though I'm no longer a member of this website, I read the forums frequently. Some of the best sports photographers in the nation are members of this website. It's filled with resources ranging from a photographer's forum and informative articles to a database of sports arena's in the U.S. I highly recommend this site to anyone interested in sports photography.