The Devteam, the Nethack Development Team.
The Devteam is a group of developers who make Nethack better and better. The group works on their own, but they do listen to the users (you can send suggestions, bug reports/fixes, and stuff to them via e-mail, and they probably also read the Usenet group rec.games.roguelike.nethack).
They keep the development more or less closed, and don't let anyone else to see the code being worked on (they used to send patches to each other via mailing list; I guess they use some sort of version control system these days, though?) They do accept new developers if the applicant shows they can do something really well and stay that way.
I think that in this case the closed work circle is better - it won't spoil any surprises. I wonder what nifty features the next version of Nethack will have?
The Devteam never gives deadlines or release dates - this way they won't miss them. Nor do they announce future features (apart of bug fixes) - this way they won't promise anything that won't get done. Nor do they tell all of the new features, so that people would have some joy of discovery. When the release is done, a lone message will appear in r.g.r.n - but it won't be lone for long... iD software and 3DRealms may have publicized the "when it's done" deadline, but no one has kept that tradition in computer game world better than the Devteam - over 15 years and still no problems.
In case you want to know the names of coders who are / have once contributed to Nethack, see the author list in the game.
Want to become a member? There are rumors on the ways to join, but doing something cool, repeatedly, is probably one way. (For example, Timo Hakulinen was interviewed once in Pelit, and he told how he got to the team: Ported the thing to OS/2, and kept maintaining it until 3.3 days...)
The Devteam thinks of everything, but they don't always produce bugless releases - but the quality and depth of the game is better than average, so I can only say they do their job very well.
The devteam doesn't have a web page, but they do maintain the official Nethack homepage at <URL: http://www.nethack.org/ >.