andLinux is a complete Ubuntu Linux system running seamlessly in Windows 2000 based systems (2000, XP, 2003, Vista, 7; 32-bit versions only). It is based on CoLinux.

Self-referential LZ file, by no other than Russ Cox.
See also https://research.swtch.com/nih, where Ken Thompsons code for a "trusting trust" attack is described in detail.
The unix v6 simulator at https://research.swtch.com/v6 does not work on Chrome or Brave under Android, but it does work on Firefox (Android 12).

SaVi - satellite constellation visualization. Simulate Iridium, Globalstar, Sirius, GPS, Teledesic and other systems, modify them, or design your own. Uses Tcl/Tk and Unix libraries; 3D viewing option requires Geomview running on X Window. BSD license.

rsnapshot is a filesystem snapshot utility for making backups of local and remote systems. Using rsync and hard links, it is possible to keep multiple, full backups instantly available. The disk space required is just a little more than the space of one full backup, plus incrementals. Depending on your configuration, it is quite possible to set up in just a few minutes. Files can be restored by the users who own them, without the root user getting involved. There are no tapes to change, so once it's set up, your backups can happen automatically untouched by human hands. And because rsnapshot only keeps a fixed (but configurable) number of snapshots, the amount of disk space used will not continuously grow. rsnapshot is written entirely in Perl.

Small SSH server and client.

How to use ZFS snapshots as backup mechanism.

Claimed to be easier to use than procmail (on the gnus newsgroup).

An early book about digital photography, from the Bell lab that brought you Unix, Plan 9 and Inferno.

Shell accounts with e-mail, web etc. for low cost.

Description of mathematical software for Unix.