Publication Style Guides on the Internet

Brief Guide to Citing Government Publications
Citing Print and Electronic Research Sources contains short examples of citations for Chicago/Turabian, MLA, APA, and CBE styles
Columbia Guide to Online Style style guide covers examples for the humanities and sciences
Elements of Style an online edition of the classic William Strunk guide to writing
Excerpts from International Standard ISO 690-2 citing electronic documents
Web Extension to American Psychological Association Style (WEAPAS) citation format for electronic documents

American Society of Agronomy/Crop Science Society of America/Soil Science Society of America The Publications Handbook and Style Manual serves as a guide for authors in preparing manuscripts and other material submitted for publication by ASA-CSSA-SSSA
Biophysical Journal - Style Guide set up in dictionary format, this style guide describes how to use various scientific terms and data elements in biological science manuscripts
CBE Documentation a short introduction to the Council of Biological Editors citation style for print publications
NLM Style Guide a very brief introduction to print citation styles recommended by the National Library of Medicine
The Official ASAE Style Guide for Refereed Publications, Monographs, and Textbooks American Society for Agricultural Engineering style guide for print publications
Scientific Citation for Electronic Sources contains an introduction to the Council of Biological Editors citation style for electronic publications

ACS Style Sheet a short guide to print citation forms used by the American Chemical Society
American Statistical Association Style Guide
CS Style Guide the online style guide for the IEEE Computer Society
Physical Review Style and Notation Guide
A Referencing Style Guide short examples from the Harvard system of referencing publications
Style guide for references from the Geological Society of America's Information for Contributors
Style Specifications for Submissions to the New York Journal of Mathematics