Chicago Manual of Style - FAQ About writing style 
Basics of Chicago Style - Live! - The differences between Chicago and MLA writing styles; tips

Wikipedia - Discussion and lists of style guides 
Library Spot - Style guides, grammar guides, and more

American Psychological Association Style - Web Extension
Associated Press - Style guide
AP Style - Basic news writing style guide for beginning journalists
AP Style - By the Royal Winsor Society for Nursing Research 
Better Writing - AskOxford.com  
Modern Language Association - Style - MLA style guidelines for writers of research papers 
California State Univ, Chico - Writing Style Guide - CSUC's basic style guide
Capital CC - Guide to Grammar & Writing - Hints on basic grammar rules 
The ComicBase Style Guide 
The Curmudgeon's Stylebook - Bill (Curmudgeon) Walsh's supplement to the AP style guide 
Elements of Style - (Bartleby.com) William Strunk Jr.'s guide to elegant writing
HyperGrammar - Structured and detailed course on literary stylistics; the U of Ottawa. 
Junket Studies - 11 Rules of Writing - Tip list for punctuation, grammar, and clear writing 
List of Style Guides - Style guides used by different fields
List: Writing Style Manuals - Half dozen mentioned
Mindy McAdams - Copyediting for Magazines - Skills and vocabulary necessary for copyediting 
U.S. Government Printing Office 
Wired Style - English Usage in the Digital Age - Correct grammar, contemporary prose for tech writing

General: The Chicago Manual of Style²
General: The Elements of Style by Strunk and White²
General: Bryson's Dictionary of Troublesome Words by Bill Bryson²
Books and general interest: The Chicago Manual of Style²
Books and general interest: The Classics of Style² 

Anthropology: American Anthropological Association's Style Guide¹ 
Biology and natural sciences: Council of Biology Editors Style Manual¹
Chemistry: American Chemical Society (ACS) Style Guide¹
Economics: Turabian (a simplified Chicago style)¹
English: MLA Handbook for Writers of Research Papers¹
History: Chicago Manual of Style; Note: some historians use MLA style.¹
Philosophy: MLA Handbook for Writers of Research Papers¹
Psychology: Publication Manual of the American Psychological Association¹
Religion: MLA Handbook for Writers of Research Papers¹
Sociology: American Sociological Association Style Guide¹  

Government: GPO Style Manual²

Medical: American Medical Association Manual of Style²
Medical: The Lancet: Formatting guidelines for electronic submission²

Newspapers : AP Stylebook²
Newspapers : The New York Times Manual of Style and Usage²
Newspapers : Wall Street Journal style guide²

Web Sites:

Web Sites: The Columbia Guide to Online Style²
Web Sites: OSNews Style Guide²

Academic

Science: ACS Style Guide (American Chemical Society)²
Scientific Style and Format: The CBE Manual for Authors, Editors, and Publishers (Council of Science Editors)²
Science: Nature - Formatting Guide²  

Social Sciences: APA style (social sciences, by the American Psychological Association)² 
Social Sciences: American Sociological Association Style Guide (American Sociological Association)²

Geoscience: Geoscience Reporting Guidelines²

Arts and Humanities: MHRA Style Guide (Modern Humanities Research Association)² 
Arts and Humanities: MLA Handbook (Modern Language Association of America)² 
Political Science Journals (American): The Style Manual for Political Science (Amer. Political Science Assoc.)²
 

Publishing Conventions: Words into Type (less scholarly, more accessible than the Chicago Manual)
Physics: Advances in Physics - Style Guide (Adobe pdf)
For a summary and comparison of academic style guides, see Style Manuals and Writing Guides by the CSULA University Library.

Computer Industry: Read Me First! A Style Guide for the Computer Industry -Sun
Computer Industry: Microsoft Manual of Style for Technical Publications

Graphic Design: An (unfinished) example of a newspaper's style guide is that of The Spokesman-Review of Spokane, Washington - MG Redesign prepared this guide when it consulted on a 2006 redesign of the newspaper.

References

1. Tutt Library
2. Wikipedia, see link for more discussion

List of International
ISO Standards

From Wikipedia

• ISO 214 — Abstracts for publication & documentation
• ISO 8 — Presentation of periodicals
• ISO 18 — Contents lists of periodicals
• ISO 31 — Quantities & units
• ISO 215 — Presentation of contributions to periodicals & other serials
• ISO 690 — Bibliographic references — Content, form & structure
• ISO 832 — Bibliographic references — Abbreviations of typical words
• ISO 999 — Index of a publication
• ISO 1086 — Title leaves of a book
• ISO 2145 — Numbering of divisions & subdivisions in written documents
• ISO 5966 — Presentation of scientific & technical reports
• ISO 6357 — Spine titles on books & other publications
• ISO 7144 — Presentation of theses & similar documents

MLA & APA Documentation (University of Delaware Writing Center)

ASCE Authors' Guidelines to Publishing (American Society of Civil Engineers)

A Guide for Writing Research Papers based on Modern Language Association Documentation (Capital Community College)

King's English (2nd edition) (H.W. Fowler)

Uniform Requirements for Manuscripts Submitted to Biomedical Journals (International Committee of Medical Journal Editors)

Writing Guidelines for Engineering and Science Students (Virginia Tech (Host), University of Illinois, University of Texas, Georgia Tech)