There are many commercial implementations of TeX. The first appeared not long after TeX itself appeared.
What follows is probably an incomplete list. Naturally, no warranty or fitness for purpose is implied by the inclusion of any vendor in this list. The source of the information is given to provide some clues to its currency.
In general, a commercial implementation will come “complete”, that is, with suitable previewers and printer drivers. They normally also have extensive documentation (i.e., not just the TeXbook!) and some sort of support service. In some cases this is a toll free number (probably applicable only within the USA and or Canada), but others also have email, and normal telephone and fax support.
Source: Mail from Richard Kinch, August 2004.
Tel: 800-808-7906 (within the USA)
Tel: +1 415-296-7550
Fax: +1 415-296-7501
Email: <a href=“mailto:firstname.lastname@example.org”
Source: Personal TeX Inc web site, December 2004
Tel: 0345 766 0340 (within the UK)
Fax: 0345 603 9443 (within the UK)
Email: <a href=“mailto:email@example.com”
MacKichan Software Inc.
19307 8th Avenue, Suite C
Poulsbo WA 98370-7370
Tel: +1 360 394 6033
Tel: 877 724 9673 (within the USA)
Fax: +1 360 394 6039
Email: <a href=“mailto:firstname.lastname@example.org”
Source: Mail from Christopher Mabb, August 2007
Note that the company Y&Y has gone out of business, and Y&Y TeX (and support for it) is therefore no longer available. Users of Y&Y systems may care to use the self-help mailing list that was established in 2003; the remaining usable content of Y&Y's web site is available at http://www.tug.org/yandy/