The author would like to thank Alison McNab for her useful CHEST-Papyrus messages on this subject. This article is partly based on them (although the author takes full responsibility for any errors, omissions &c. &c.
A widely recommended reference for this is Li, X., & Crane, N.B. (1996). Electronic styles: A handbook for citing electronic information (Revised edition). Medford, NJ: Information Today. [ISBN: 1-57387-027-7, $19.99] This covers both MLA and APA styles; there are some basic examples from the new book in Nancy Crane's pages unlike their earlier book, Li, X; Crane, NB; Electronic Style: A Guide to Citing Electronic Information; Mecklermedia, Westport; 1993, which only covers the APA standard. Bournemouth University Library have prepared a very nice presentation of the ideas in this earlier book, but of course, like the book, it is limited to APA/Harvard style.
Online! is a new book, published in 1998, by Andrew Harnack and Eugene Kleppinger. There is a supporting Web page which is said to include chapters 5 to 8. This presents a wider range of styles than Li and Crane: APA, MLA, Chicago Manual, and CBE. The authors have also written an article in Ariadne.
The International Federation of Library Associations have a useful page of links, UCL Library reckon it is probably the best "Guide to guides". I have also found the BUBL and Yahoo! lists useful. At the time of writing, the all pages listed described approaches to extending MLA, APA, and Harvard styles. In addition, the University of Alberta libraries has an extended list of print publications in the area. The Naval Research Laboratory at Stennis Space Center points out that Internet RFC 1738 specifies a format for citing Internet URL's.
Unfortunately I have not come across any suggestions for those of us who use formats similar to the ACS ones. The draft ISO standard for bibliographic references to electronic documents adopts the APA practice of indicating the medium, placing it within square brackets; as I understand it, this is done with all APA references, not just electronic ones. For my PhD thesis I adopted this approach, solely for the electronic references, to give references like these:
1. Ramsey, N. NOWEB, (Version 2.5a), [Computer Program]. Available FTP: ftp.tex.ac.uk Directory: /ctan/tex-archive/web/noweb/DOS 1994.
2. Kent, B. Hostaflon, [Discussion], [Online]. Available e-mail: USENET Newsgroup: sci.polymers 28 October 1994.
Bernard Treves BrownSCEAS, University of Manchester. United Kingdom.