Knuth was making a particular point about the capabilities of TeX when he defined the logo. Unfortunately, many believe, he thereby opened floodgates to give the world logos such as AMS-TeX, PicTeX, BibTeX, and so on. Lamport invented LaTeX, and marketing input led to the current logo LaTeX2e.
The common people don't have to follow this stuff wherever it goes, but, for those who insist, a large collection of logos is defined in macros/eplain/texnames.sty; the METAFONT logo can be set in fonts that LaTeX2e knows about (so that it scales with the surrounding text) using the package macros/latex/contrib/supported/mflogo
For those who don't wish to acquire the `proper' logos, the canonical
thing to do is to say AMS-\TeX{} (AMS-TeX) for AMS-TeX,
Pic\TeX{} (PicTeX) for PicTeX, Bib\TeX{} (BibTeX) for
BibTeX, and so on.