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 a whole range of rather silly 'bumpy road' logos such as AMSTeX, PiCTeX, BibTeX, and so on, produced in a flurry of different fonts, sizes, and baselines - indeed, everything one might hope to cause them to obstruct the reading process. In particular, Lamport invented LaTeX (silly enough in itself, with a raised small 'A' and a lowered 'E') and marketing input from Addison-Wesley led to the even stranger current logo LaTeX2e, which appends a lowered single-stroke Greek letter epsilon.
Sensible users don't have to follow this stuff wherever it goes, but, for those who insist, a large collection of logos is defined in the texnames package (but note that this set of macros isn't entirely reliable in LaTeX2e). The Metafont and MetaPost logos can be set in fonts that LaTeX2e knows about (so that they scale with the surrounding text) using the mflogo package; but be aware that booby-traps surround the use of the Knuthian font for MetaPost (you might get something like 'META O T'). You needn't despair, however - the author himself uses just 'MetaPost'.
For those who don't wish to acquire the 'proper' logos, the canonical
thing to do is to say AMS-\TeX
for AMSTeX, Pic\TeX
for PiCTeX, Bib\TeX
for BibTeX, and so on.
While the author of this FAQ list can't quite bring himself to do away with the bumpy-road logos herein, he regularly advises everyone else to...