The Piazza went down this Weekend, and for a while it looked like the site had run into some serious problems. Fortunately, due to swift action from its webmaster, the site is back up again fully intact today.
Although there are many great web forums for D&D fans, The Piazza is still the best forum focusing on multiple D&D settings. The Mystara and Spelljammer communities are the most active there, but many other worlds are also being discussed. The list of settings with their own forum there was greatly expanded for the 5 year anniversary earlier this year. Thanks to Ashtagon for the hard work in maintaining this site over so many years now.