Infocom has the three
Original Zork text adventure games for download for free.
Also while I've seen ADOM and Nethack mentioned I have yet to see anyone mention:
Moria (warning: this game is no longer maintained) or
Angband or any of its
Variants
These games are available for windows(except for moria), Linux or Mac.
P.S. Diablo was based of the Rogue/Moria/Angband tree, not the Hack/Nethack tree :P