NetHack's predecessor, Hack, was originally written by Jay Fenlason et. al. Apparently, this was included on a Usenix tape, probably in the early 1980s, but I haven't managed to get a copy yet.
However, thanks to Kevin Otto, we do have one surviving DOS binary:
Note: For a long time, I believed that this was a variant of the original hack due to the inclusion of "Slak was here!" in the version number, but it turns out that this tag was used by Jay himself. On the other hand, hack was originally written on a UNIX system, so this DOS binary has obviously been ported by somebody unknown.
Julie Bresnick's interview with Jay Fenlasson gives some interesting background to the creation of hack.
A copy of the original READ ME file was preserved in AB Hack 1.0 (see below).
Andries E. Brouwer produced a number of enhancements which were distributed via net.sources and net.sources.games as shar packages and patches. These have been re-constructed as accurately as possible (some parts have been lost) and re-packaged into tarballs as follows:
From this Donald G. Kneller produced a number of versions of PC Hack. The following versions are mentioned in the history file for NetHack: 1.01e, 1.03g, 3.0, 3.2, 3.51, 3.6.
R. Black created an Atari version called ST Hack based on PC Hack 3.51. I don't have any source for this.
Older versions of NetHack were distributed via comp.sources.games in shar format. These have been re-packaged into tarballs as follows:
Newer versions of NetHack are distributed in tarball and diff format:
NetHack++ / NetHack-- was written by Tom Proudfoot and Yuval Oren.
NetHack Plus was written by Stephen White who released a number of updates:
Slash was written by Tom Proudfoot who released six versions:
Slash'EM was started by Warren Cheung as a merger of Slash 6 and the wizard patch. It is now maintained by the Slash'EM development team.