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.
Richard J. KinchSource: Mail from Richard Kinch, August 2004.
TrueTeX Software
7890 Pebble Beach Court
Lake Worth FL 33467
USA
Tel: +1 561-966-8400
Email: kinch@truetex.com
Web: http://www.truetex.com/
Personal TeX IncSource: Personal TeX Inc web site, December 2004
725 Greenwich Street, Suite 210
San Francisco, CA 94133
USA
Tel: 800-808-7906 (within the USA)
Tel: +1 415-296-7550
Fax: +1 415-296-7501
Email: sales@pctex.com
Web: http://www.pctex.com/
MicroPress IncSource: Mail from MicroPress, Inc., July 1999
68-30 Harrow Street
Forest Hills, NY 11375
USA
Tel: +1 718-575-1816
Fax: +1 718-575-8038
Email: support@micropress-inc.com
Web: http://www.micropress-inc.com/
Dr Christopher Mabb
Scientific Word Ltd.
49 Queen Street
Peterhead
Aberdeenshire, AB42 1TU
UK
Tel: 0845 766 0340 (within the UK)
Tel: +44 1779 490500
Fax: 01779 490600 (within the UK)
Email: christopher@sciword.demon.co.uk
Web: http://www.sciword.demon.co.uk
MacKichan Software Inc.Source: Mail from Christopher Mabb, May 1999
600 Ericksen Ave. NE, Suite 300
Bainbridge Island WA 98110
USA
Tel: +1 206 780 2799
Fax: +1 206 780 2857
Email: info@mackichan.com
Web: http://www.mackichan.com
Blue Sky ResearchSource: TUGboat 15(1) (1994)
534 SW Third Avenue
Portland, OR 97204
USA
Tel: 800-622-8398 (within the USA)
Tel: +1 503-222-9571
Fax: +1 503-222-1643
Email: sales@bluesky.com
Web: http://www.bluesky.com/
Radical Eye SoftwareSource: Mail from Tom Rokicki, November 1994
PO Box 2081
Stanford, CA 94309
USA
This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=commercial