Mentalepsy: The Might & Magic collection is one of the best values on GOG, and will keep you busy for a while.
Soggy_crackers: Actually a friend of mine just picked that up on GOG here... seems something that could be a good bet! I've played the earlier ones, but what is the multiplayer like? Seems like a good game to have some friends over and a case of beer.
Might and Magic has no multiplayer.
I'd agree that M&M might be a great value for $-to-time-to-beat, but not $-to-time-to-enjoy. I've blown plenty of 4 to 8 hours stints on M&M, don't get me wrong. But Neverwinter Nights has it beat hands down by a good long shot.
DDO has been a blast, but I'd warn to stay away from the Lord of the Rings Online free-to-play game.
Also, if you want to lose your life for a good a few years for free, play City of Arabel or another good story-rich multiplayer server on Neverwinter Nights. Just beware of the lag monster. It kills far more than Beholders.
Another good cheap game that I've spent countless hours on is Mount and Blade. It's definitely not for everyone. But holy smokes I've wasted my life on that game. When I die, I'll probably be ashamed of my life's pie chart of spent time. NWN and Mount and Blade will get their own pie pieces.