Nethack is good !! You can download it at
www.nethack.org for free. On the first glance it seems a little bit like diablo (at least the gaming prinziple, of course not the graphics) but on the second glance there are lots of jokes and gimmiks in the game and it makes big fun to discover them all.
I could write pages of things to dicover, but try out the game by yourself.
BTW, there are several modes of the game. The LINUX and Windows-versions have a graphics interface, other versions are only ascii.
Chris