Crawl has a number of variants changing aspects of gameplay, not the least of which is Dungeon Crawl Stone Soup. Many of them are still actively developed. This page has a list of variants (of DCSS) which are or were available for play on online servers, along with some interesting features of each.
BcadrenCrawl offers dual wielding of weapons; a silent spectre species that has a silence aura it can cast through; a goblin species with low stats, high apts, and extra invocations power; some new backgrounds (including Demigods reworked as a background); and god reworks including a feature where Hepliaklqana ancestors differ based on your own species.
More information here. Based on 0.22. Available on CPO, CKO, and CXC.
Based off the community driven X-Crawl, the author of bcrawl believes that vanilla development has stagnated, so bcrawl has a number of experimental features intended to shake the game up. The Dungeon and many branches are shortened, and instead of random traps by exploring, going up stairs has a chance to teleport you. On the converse, it also does not include many of the more gameplay-changing streamlinings of DCSS, such as the removal of food and curses (though all items are identified if you have a rune).
Instead of global changes, bcrawl mostly features reworks to many backgrounds, species, gods, etc., with many balance changes especially to spells. It also adds six species, an Archaeologist background, and an alternate version of Reaver.
More information here. Scoring here. Based on 0.21, but includes many future changes. Available on CPO, CXC, CKO, CUE.
Bloatcrawl focuses on adding new species and backgrounds, adding over a dozen of each, including backgrounds that start worshipping Jiyva, Vehumet, Makhleb, Cheibriados, Qazlal, Ashenzari, and Nemelex.
More information here. Based on 0.23.
Bloatcrawl2 is the sequel of bloatcrawl, featuring even more species and backgrounds (for a total of 69! of each). Additionally, players can choose to add undead characteristics (Mummy, Ghoul, or Vampire) to any race, as well as alter a few other settings, before the game starts.
More information here. Based on 0.25. Available on CKO.
DCSS Circus Animals
DCSS-CA reworks many species, and brings back some removed and experimental species such as Lava Orcs and old Djinn. It has optional new modes for getting experience, such as gaining experience levels upon reaching new floors rather than killing monsters. It restores the unfinished experimental Enchanted Forest and Dwarven Halls branches. It adds difficulty levels and a stamina system where your character can temporarily exert themselves to do more damage, have better spells, or move quickly.
More information here. Based on 0.19.
You cannot gain any experience levels or skills, and every enemy is a gnoll. Gnoll sergeants and shamen sometimes turn up in Zot, and the final boss is Grum.
More information here. Based on 0.19.
Other "all enemies are X" variants
gnollcrawl.tk also offered boggartcrawl, OOFcrawl, and lichcrawl, all of which have similar mechanics.
Features are added to Gooncrawl based on democratic voting amongst the readers of Something Awful, a long-running Web forum. Gooncrawl undoes many removals unpopular with those readers, such as High Elves, Death Knights, removed spells, wands of hasting, and mutagenic chunks. It also reworks some species, adds some new features such as new spells, and implements a "permabuff" system where spells such as Regeneration need not be repeatedly recast.
More information here. Scoring here. Based on 0.22.
Hellcrawl aims to streamline the Crawl experience. It removes things which the author feels are unnecessary, like the Lair, player ghosts, and identification. There are no stairs up; you must continue to descend through the dungeon, and once you have the Orb of Zot you attempt to escape via Hell. In turn, it reworks some species, gods, and monsters. It also turns out that many things that were removed in Hellcrawl - like physical spellbooks, food, and curses - were eventually removed from Stone Soup, too.
More information here. Based on 0.19. Available on CPO, CKO, LLD.
Design goals: "Development will be focused on adding content rather than deletion. For adding content, even if a balance problem is seen, the goal is to add it first. Restoring old content is also one of the main goals while maintaining the current system." The players can vote on which features they would like to see added or changed.
This variant brings back Pakellas, Fulsome Distillation and Evaporate, Singularity, rods, disc of storms, mutagenic corpses, and top-tier wands. Players have access to more inventory space via a new misc item, a bag. God reworks include numerous improvements for managing and levelling up orcish allies; Hepliaklqana's battlemage ancestor can cast Fireball and Fire Storm; Yredelemnul, Trog, Jiyva, and Zin can brand weapons with thematically appropriate egos. Also, there are two new gods: a god of alchemy and a god of reflections. Finally, Kimchi adds homunculus, hydra, meliai, and several other species, four new backgrounds, and a whole bunch of new spells.
The change log can be found here. Based on 0.24. Available on CWZ.
Stoat Soup was started because of dissatisfaction with the removal of monster spawns over time in 0.21; it restores these spawns. It restores the removed High Elf and Mountain Dwarf species and adds a new Faerie Dragon species intended to be a straightforward option for spellcasting. It implements an independently developed permabuff system. Player ghosts in Stoat Soup are not found in vaults, but appear to seek revenge on whatever killed them, potentially helping the current player.
More information here. Scoring here. Based on 0.22. Available on CPO and CKO, and (console-only) crawl.montres.org.uk.
Yiuf's Home for Lost and Forgotten Species re-adds some removed species and adds one new one, the Skeleton. Some mutations are reworked, and there are some other balance changes.