Hardfought
Hardfought is a nethack community with its own servers, similar to NAO. Hardfought currently hosts three servers in Vermont, London, and Sydney.
Hardfought has its own IRC channel on freenode, #hardfought. This is also linked with the #nethack-hardfought channel on the Roguelikes discord. The Beholder bot announces important events, deaths, and ascensions, and Pinobot can be used to find information on enemies across NetHack variants.
Variants hosted
NetHack 3.4.3, NAO version
NetHack 3.6.1, HDF version, official release with public server based enhancements. Bugfixes from the upcoming 3.6.2.
GruntHack, with bug fixes and UI enhancements
SporkHack, with bug fixes and UI enhancements
xNetHack, 3.6.1 based variant
SpliceHack, 3.6.1 based variant
More to follow soon...