My own spoiler set. The monster manual contains of a description of every monster with details of its abilities and attacks.
Slash'EM (Super Lotsa Added Stuff Hack - Extended Magic) is an actively maintained variant of NetHack. It adds more monsters, more objects, more dungeon levels, more roles and more races. You can make use of shopkeeper services, class specific techniques, invisible objects and gypsies if you feel lucky.
The latest stable version of Slash'EM is based on NetHack 3.3.1 (which itself borrows some ideas from earlier versions of Slash'EM). It comes with a GTK windowing interface based on Issei Numata's JNetHack and with 32x32 and pseudo-3D tile sets based on Mitsuhiro Itakura's tile archive and Wingnut's AllegroHack.
Nh:TNG is an old variant based on NetHack version 3.1.3. I mirror it here because it no longer has a home of it's own.
Ever wanted to see how NetHack has developed over the years? All versions of NetHack publically released are available here.
Configure the game the easy way.
See also, Tom Wilson's auto-nethack package.
Compare tiles available from the various NetHack variants and window ports.
32x32 and Pseudo-3D tilesets and GTK interface for NetHack 3.3.1
My collection of patches and related links for NetHack and its variants.
NetHack is, of course, single-user. The dev-team have made it quite clear that they do not intend to change this (see here for details). It is however possible for players to interact in a limited way via bones files. If you are the only person playing on your machine then the following links may prove useful: