NetHack's cult status combined with its well-written, open-source code led to many mods being created over the years. Most of them are referred to as 'variants' and they modify the gameplay in various ways - from introducing new classes, items and monsters to adding new objectives that must be completed before players can finish the game. There are many variants and most of them are either discontinued (although if a popular variant is abandoned by its developer, someone will probably make a new one based on it) or come in form of simple patches so only the notable ones will be listed here.

The other side of modding NetHack are attempts at making alternate graphical interfaces for it. Changes applied by those can be simply cosmetic (e.g. a pseudo-3D view instead of traditional tiles) but some of them also significantly modify how the game is controlled (which can be useful to those players who can't get used to vi-style controls and don't have a numpad).

SLASH'EM Extended

SLASH'EM Extended allows for rather creative race/class combinations.

Developed by Amy aka Bluescreenofdeath, this is the last one in the very long line of popular NetHack variants (including Nethack++, Slash and SLASH'EM). SLASH'EM Extended greatly expands upon the original game. There are more races and classes now, the Gehennom is less boring and the bosses provide a bigger challenge. There are also many more special rooms added, along with some additional dungeon branches. It also aims to rebalance the game while reducing the frustration (it even allows you to continue in explore mode after you die).

While SLASH'EM Extended is a very good variant - especially for those who find vanilla NetHack too easy and would like to try their hand at playing a challenge class like convict (starts with an iron ball chained to his leg and a pet rat, the shopkeepers are hostile towards him), it suffers a bit from lack of focus. It's as much a mod as it's a collection of other mods, and their quality is rather varied. It also goes a bit overboard with references as even in the world of NetHack the cyborgs, pokemon and jedi knights feel out of place.

SLASH'EM Extended can be downloaded here.

UnNetHack

UnNetHack uses ASCII-based tiles

The philosophy behind UnNetHack seems to be completely different from that of SLASH'EM Extended as Patric Mueller's variant is more focused on redesigning the game than on adding to it. There is some added content (and it sometimes comes from other variants and patches) like an ability to play as a vampire, the black market level or some new monsters (often coming from unlikely sources, although UnNetHack's weeping angels and uranium imps still make more sense in NetHack's context than SLASH'EM Extended's metroids) but in general, UnNetHack aims to be a different game rather than a bigger one.

As NetHack can get a bit predictable after a while (or so claim the expert players), UnNetHack tries to make it more random by introducing more variation to the levels (with new types of special rooms and more variation in the fixed ones) and making sources of wishes harder to find. It also improves some of the previously useless artifacts like Heart of Ahriman, removes the annoying scroll of amnesia and fixes some bugs.

UnNetHack can be downloaded from its SourceForge page.

NetHack 4

Title screen of the official NetHack 4 server

As the title implies, Alex Smith's NetHack 4 is an attempt to continue the official game's development without the original NetHack DevTeam (who didn't release an official new version since 2003). As such, it's aimed more at the new-to-intermediate players than at the veterans who usually play variants.

While NetHack 4 might not surpass the quality of the official releases, it has a great potential. While NetHack purists will probably dislike the replacement of D&D-style armor class with defence stat and addition of automated food safety warnings, everyone can appreciate the great modernization of the game's interface while staying true to ASCII-based roguelike style. The game is really easy to navigate now without the need to memorize keyboard commands, and the new auto-explore command greatly reduces the tedium, especially in Gehennom.

NetHack 4 can be downloaded from the official website.

Vulture

Vulture intro

Vulture isn't really a NetHack variant as it doesn't change anything in the core game. It simply presents an already existing game in a nice isometric perspective, allows for full mouse control and adds some music and sound effects (although they have their issues - for example, the sounds of cat meowing and dog barking are triggered respectively by the strings of letters 'cat' and 'dog' appearing in the message buffer, which means that you'll hear them when picking up a 'dog-eared spellbook' or when one of your scrolls 'catches fire and burns'). It also adds a short introduction describing how your character found himself in the dungeon, but that isn't really too interesting.

The graphics in Vulture aren't bad looking, but they're still static images just like tiles in the original game. Additionally, skewed perspective makes it difficult to play this game with a keyboard. Other than that though, Vulture looks and plays very well - especially for people more used to Infinity Engine (Baldur's Gate, Planescape: Torment etc.) and similar RPGs than to ASCII or tile-based roguelikes.

Vulture is available at Desura. There is a free version available as well as 10$ 'fanboy edition', although the paid version doesn't offer anything other than beta access right now (there are plans for things like achievements and cloud saves but those are yet to be implemented).

NetHack 3D

Typical NetHack 3D visuals

NetHack 3D is a very ambitious project to recreate NetHack in true, animated 3D using Irrlicht Engine. It features models for every character class and many monsters, full mouse support and sound effects. Like Vulture, it's based on the old Falcon's Eye project and shares many of its quirks (including sound effects triggered by the message buffer) and some of its menu graphics.

Unfortunately, the game was released in an extremely unfinished state (the current version is called 1.1 despite clearly being an early alpha). Models occupying the same spot on the map (like a character standing on an altar) clip through each other, mouse control is clunky and buggy, many functions of the keyboard interface don't work properly, the sound effects are annoying and the textures for floors and walls look less like a dungeon and more like a public toilet. The project could have been something interesting but seems to be abandoned and as it stands is glitchy, bizarre and has a strange, claustrophobic feeling to it. It is something unique though: probably the only example of NetHack kusoge.

NetHack 3D is available at SourceForge, but it's probably the worst way to experience this game. It's a must for those fascinated by buggy and kind-of creepy (or full-on terrifying if you play as a kinght and manage to ride a horse) low-poly tech demos though.