Crawl has a number of variants changing aspects of gameplay, some of which are still actively developed. This page has a list of variants which are available for play on online servers, along with some interesting features of each. As well as the regular public servers, many variants are offered on gnollcrawl.tk.
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.
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, many backgrounds and species are reworked (and Archaeologist and Reaver backgrounds are added along with Fairy, Sand Dwarf, Oni, Skeleton and Djinni species), some gods are reworked, and there are many balance changes especially to spells.
Bloatcrawl focusses 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.
DCSS Circus Animals
DCSS-CA reworks many species, and brings back some removed and experimental species such as lava orcs and djinni. 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.
Other "all enemies are X" variants
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.
Hellcrawl aims to streamline the Crawl experience; it removes things like curses, food, and item identification which the author feels are unnecessary. 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. It reworks some species and monsters, and was the first variant to implement a permabuff system (from which Gooncrawl's system is derived). Some important Hellcrawl features such as the spell library are now in vanilla Crawl.
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.
Yuif'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.