For PCs, running MS-DOS or OS/2, EmTeX (by Eberhard Mattes) offers LaTeX, BibTeX, previewers, and drivers. It is available as a series of zip archives, with documentation in both German and English. Appropriate memory managers for using emTeX with 386 (and better) processors and under pre-'9x Windows, are included in the distribution. (EmTeX can be made to operate under Windows, but even back when it was “current”, such use wasn't very actively encouraged.)
A version of emTeX, packaged to use a TDS directory structure, is separately available as an emTeX “contribution”. Note that neither emTeX itself, nor emTeX-TDS, is maintained. Users of Microsoft operating systems, who want an up-to-date (La)TeX system, need Win32-based systems.
For PCs, running MS-DOS, a further option is a port of the
Web2C 7.0 implementation, using the GNU
compiler. While this package is more recent than emTeX, it
nevertheless also offers a rather old instance of (La)TeX.
For VAX systems running OpenVMS, a TeX distribution is available CTAN, but is almost certainly not the latest (it is more than 10 years old). Whether a version is even available for current VMS (which typically runs on Intel 64-bit processors) is not clear, but it seems unlikely.
For the Atari ST and TT, CS-TeX is available from CTAN; it's offered as a set of ZOO archives.
Amiga users have the option of a full implementations of TeX 3.1 (PasTeX) and MetaFont 2.7.
It's less likely that hobbyists would be running TOPS-20
machines, but since TeX was originally written on a DEC-10
under WAITS, the TOPS-20 port is another near approach
to Knuth's original environment. Sources are available by anonymous
ftp from ftp://ftp.math.utah.edu/pub/tex/pub/web