NetHack

Nethack is a classic Rogue-like hack and slash game. Diablo is a real-time H&S game heavily based on the turn based Nethack game; however it falls far short of all the great ideas embodied in nethack. Very cool features of this game include, stores, zoos, ghosts of previous attempts, luck, alignment bonuses, polymorphing, lycanthropy, wishing, enchanting and recharging magical items, dipping items in potions, gaining intrinsic abilities by eating certain creatures or being blessed by your diety, digging, towns, pets and much more. Nethack can be played with a downloaded interface including some graphic ones or online via Telnet.

Other Links : NetHack on Wiki
Popular Telnet Nethack Server
Public Telnet Nethack Servers
Extensive List of Nethack Websites