Slash'EM

Slash'EM (an abbreviation for Super Lotsa Added Stuff Hack - Extended Magic), is a variant of the roguelike game Nethack with many extra features, monsters, and items. Several Nethack features, such as the Monk class, conduct (voluntary challenges) and the Sokoban levels, were first developed in Slash'EM and then incorporated into "vanilla" Nethack.

The main dungeon in Slash'EM is much longer than in Nethack, and it incorporates many new special levels. One of the most exciting is the Guild of Disgruntled Adventurers, which is full of "player monsters" (archaeologists, barbarians and so forth), often equipped with offensive wands. Gehennom is much shorter than in Nethack, reducing the tedium of trekking through repeated maze levels. It also has a special level for every demon lord.

External links:
http://slashem.sourceforge.net/ - The Slash'EM home page
http://www.statslab.cam.ac.uk/~eva/slashem/ - A collection of spoilers on the differences between Slash'EM and Nethack.