A new challenger Approaches!
Welcome to RPOL! This is the best place I've found online for play by post. If you decide to branch out beyond D&D, there are quite a broad range of systems with active games here (perhaps you can give Pathfinder a try, for example), but you'll find more D&D games here than you'll know what to do with. =)
Games lagging and dying are a problem here, but we have a ton of games that have been going for years. Don't be shy about joining ongoing games that are looking for new players, either. I was lucky to get in on a game that had already been underway for a year, and nine years later, I'm still playing that game (albeit with a different character now).