Style Manual: for authors, editors and printers
This is the quintessential guide to writing well in Australian English. It is mandatory for all Australian government departments but most businesses and editors in the know use it as their 'style bible'. The link above will take you to the government site where you can purchase this book. A must.

The European Union's English Style Guide
An online PDF version of the EU's guide to writing British English.

The Good Writing Guide
This is our favourite document. Written by an academic from the University of Aberdeen, this downloadable word document will walk you through everything you need to know about writing a good essay or thesis. In doing so, it guides you through the fine art of writing well in general. This is a must for all students.

University of Memphis Style and Manual Guides
A comprehensive list of (good) links to American English style guides.

The Economist's English Style Guide
This useful guide is based on the style book that is given to all journalists at The Economist.

The Literary Encyclopaedia English Style Book
An online guide to the writing of scholarly English.

The Elements of Style
An online reference by William Strunk Jr outlining the principal requirements of plain English style, concentrating on the rules of usage and principles of composition most commonly violated.