Neverwinter Nights
Neverwinter Nights, created by Obsidian Portal, is a CRPG that is essentially D&D (Forgotten Realms) as a computer game. It allows play using all core material and a number of other sourcebooks.
That said, it's pretty good. The one issue is that it takes up a relatively large amount of space to install. Players can create custom modules, often leading to modules containing large amounts of treasure.
The most active server that is still running is called Higher Ground
Looks to be somewhat interesting as an MMO, if interested in it, have a look at TB's video [1]