Variant

Where are the NetHack variants?

Best known is SLASH'EM. That game and its ancestors (NetHack--, NetHack Plus, SLASH, Wizard Patch) are now part of vanilla NetHack's game history, because of the reuse of certain SLASH'EM code in NetHack 3.3.0. SLASH'EM puts more effort into killing the hero; plenty of games end at Dlvl1. There are some new monsters and new magic (thus SuperLotsaAddedStuffHack - Extended Magic). The dungeon features several new one-level dungeon branches; three new portals lead to quests for artifact keys. The player also has a larger choice of races and roles than in vanilla.

Other than that, so many players are busy trying to ascend in vanilla that variants have not much attention.

One of the newest variants out there is NetHack brass. Coming from Japan at 29 September 2004, it is a variant of NetHack 3.4.3 that remains mostly unknown among NetHack players who do not speak Japanese. It adds no new races or roles, but has a few new items and a redesigned Gehennom. Its major features include the new #skills system and tweaks to how heroes gain experience levels and attributes.

There was a variant called NetHack: The Next Generation of NetHack 3.2.2, but it has mostly fell into disuse. TNG is mostly remembered for the Geek class and the Douglass Adams themed monsters.

Where are the other NetHack variants?

You probably want a patch. Some patches make only interface changes, but others adjust aspects of the gameplay. TNG was a patch. If you want major changes, two choices are the Lethe patch against NetHack 3.3.0 and the biodiversity patch against NetHack 3.4.3. The Lethe patch adds several fixed but interesting levels to Gehennom, the chief feature being the river that causes amnesia; it also adds some new monsters and installs Cthulhu as the high priest of Moloch. The biodiversity patch adds several monsters to the game, adjusts some monster behaviors and adds a few other features.

You can also select patches from the Patch Database http://bilious.homelinux.org or L (person)'s collection http://www.greyfire.org/~l/nethack.html – beware of conflicts and rejected hunks if you apply multiple patches!