Gumband is an Angband variant maintained by Joseph William Dixon, based primarily on the code from ZAngband 2.1.1c. Gumband replaces many of the references to The Chronicles of Amber by Roger Zelazny with references to Michael Moorcock's Eternal Champion books.
Gumband is frequently considered to be 'munchkin', as it is balanced in ways that differ from most other Angband variants. In Gumband, a player can expect to get more powerful items, but also face baddies that hurt more than their counterparts in Vanilla Angband.
Gumband is perhaps most famous for its implementation of astral mode. With an astral character, you start down at the bottom of the dungeon (level 97 or so) and walk up before going back down to kill the big evil bosses - Arioch, Xiombarg, and Mabelrode
Astral is popular because of the speed that you can play - it's not uncommon for a reasonabily skilled player to jump 20 levels or more, something quite unheard of in most other variants.
(the above information might have been accurate a few years ago. It isn't now: Gumband is maintained by some other fellow, etc.: <http://www.thangorodrim.net/gumband.html>