Try nwvault (like previously mentioned) there are a ton of modules but quite a few of them are shite. The Penultima series are really good (if a bit different) as well as the Spires of Ravenloft. I've played both and they are excellent single player modules. Despite the garbage, there are alot of well designed mods on nwvault. Read the synopsis and user reviews before you download a module and you should do fine. It's hard to find a serious group of players to game with not to mention a DM, so I pretty much only play single player, but the modules give you a never ending supply of gaming. That's the beauty of the game.