Any of the Gold Box series. You oldschoolers know what I'm talking about. ESPECIALLY Eye of the Beholder and Eye of the Beholder II (forget III, that was a piece of crap), as well as Dungeon Hack and Stronghold (1990).

Daggerfall. If it wasn't for this game's legions of fans (including myself) on uesp.net and TES: The Essential Site, Morrowind would never have even existed.

Fallout 1 & 2. What else is there to say? If you haven't played the Fallout series, you cannot consider yourself a real role-player.

Baldur's Gate I & II (plus expansions). As above, if you haven't played the Baldur's Gate series, you cannot consider yourself a real role-player.

Castle of the Winds. A Rogue-like game for Windows 3.1 (but works perfectly fine in XP) that was once shareware, but released years later as freeware.