Morrowind with it's expansions is great for a freeform nonlinear RPG.

Neverwinter Nights is recommended for its multiplayer (mostly, since the singleplayer isn't very good at all IMHO).

The Baldurs Gate series are excellent, and so are the Fallout series (not Tactics), if you haven't played them yet.

A fairly new RPG is Temple of Elemental Evil, which after patching it, is a very very nice implementation of the D&D 3.5 ruleset and combat is great. It is however just a hack-and-slash, not so storydriven as the Baldurs Gate series, more like Icewind Dale (far better though).

Star Wars: Knights of the Old Republic, is a star wars themed RPG, based loosely on the D&D ruleset, very linear, very strong storyline and all in all a great game.