Dungeon Master, Eye of the Beholder, Lands of Lore. True gems indeed. Grimrock captures their playstyle, look and feel quite accurately, with a perfect touch of what technology can do to improve them nowadays.
I agree sidney666, Mandate of Heaven was very good, but it was a more technologically evolved implementation of the game concept. Grimrock plays more like the ones above.
BTW my all time favorite game is Anachronox, followed by Elvira II.