For me, Neverwinter Nights is it - the game offers solo play, not just with the 'Original Campaign' that comes with the game purchase, but (especially) the free modules you can download that were built by users. Some are awesomely good, and there are tens of thousands out there. More the game supports having a DM 'run the game' so you can get a very D&D experience of playing in a small party on a DM'd adventure as an option. Many multiplayer servers exist. My own server (Argentum Regio) is built to allow solo or party play of DM'd or DMless adventure events. It is a very flexible game - note we have modified the game to allow things like flight, swimming, climbing, jumping, squeezing through tight spaces, we added over 100 spells from D&D (Wish, Wall of Ice, Blink, more)... the feature list includes tracking, hiding in things and so much more. Additional info at http://argentumregio.wikia.com/wiki/Main_Page?&cb=8705