General

• Merriam-Webster's Dictionary of English Usage (Google Books)
• The Elements of Style by William Strunk, Jr. (New York, 1918)
• The American Heritage? Book of English Usage (Bartleby.com)
• Rhetoric and Composition from Wikibooks (available in PDF)
• The Purdue Online Writing Lab (OWL)
• Guide to Grammar and Writing, sponsored by the Capital Community College Foundation (Connecticut).
• Jargon Buster (AskOxford).
• The Forest of Rhetoric (''silva rhetoricae''), by Dr. Gideon Burton of Brigham Young University.
• A Plain English Handbook: How to Create Clear Sec Disclosure (Google Books)
• A Dictionary of Modern English Usage by H. W. Fowler (Google Books). Note: it can't really be considered as ''modern'', as this is 1926 publication, but guess it's a good reference anyway.

International Institutions

• Modern Humanities Research Association (PDF)
• World Health Organization (PDF)
• Interinstitutional style guide of the European Union

British English

• English Style Guide by the European Commision (PDF)
• BBC News style guide (PDF)
• The Guardian (also in PDF)
• The Times
• The Economist

American English

• U.S. Government Printing Office
• American Physiological Society
• American Psychological Association (APA) Guide Basics Summary (PDF)
• American Antropological Association (PDF)
• American Society of Newspaper Editors
• University of Colorado
• Indiana University
• University of Minnesota
• Penn State University

Canadian English

• The Canadian Style: A Guide to Writing and Editing (Google Books)

Australian English

• Queensland Government Style Guide

Other references

• Dictionary.com style guide
• Wikipedia manual of style
• National Geographic

Technical

• IEEE Standards Style Manual (Download in PDF)
• IEEE Computer Society Style Guide
• Online Technical Writing by David A. McMurrey
• Technical Writing A-Z: A Commonsense Guide To Engineering Reports (Google Books)

Citation

• Citation Styles Handbook: APA
• Cite source by Trinity College Library (summary of the most important citation standards).
• Stylewizard (assistant for creating correct APA citations).

More info

• Style guide (Wikipedia)
• Style Manuals & Writing Guides by Penn State University (collection of links).
• Grammar, usage and style by Bob Drudge (collection of links).
• The EServer Technical Communication Library (index of published works for professional, scientific and technical communicators).

print