Australian Style Manual  An excellent and affordable guide. This has long been the standard Australian work of reference for authors, editors and printers in both public and private sectors. Available from Government Info Shops in all capital cities (Phone 132 447). The complete version 5 manual is available in PDF format at this website.

The Chicago Manual of Style  The Chicago Manual of Style has set the editorial standard since 1906, providing consistent, systematic guidelines for writers, editors, proofreaders, indexers, copywriters, and publishers. The all-time best style manual (as long as you can filter out the American bits).

Yale C/AIM Web Style Guide  This guide is considered by many to be the primary standard for the development of content and applications for the World Wide Web. You can decide for yourself. However, with more and more documentation being produced for onscreen use, these principles have become relevant for all forms of documentation. The complete guide is available at this site.

Common Errors in English  This is the best resource on the web for common errors in English. It also includes extensive set of related links, commonly misspelled words and a section on Non-Errors - those usages people keep telling you are wrong but which are actually standard in English.  

Robert Bly  Bob Bly is an independent copywriter and consultant with more than 20 years experience in business-to-business, high-tech, industrial, and direct marketing. His website includes numerous free articles on marketing, business skills and personal development. You'll  find some amazingly helpful information here. I also highly recommend his books "The Copywriters Handbook" and "The Online Copywriter's Handbook".

Plain English Saves Time and money  Plain English presents information in a way that readers can read, understand and act upon with a single reading. They read it quicker, understand better, and remember longer. 
Independent research from companies and governments around the world continues to demonstrate that clear writing in plain English produces significant business benefits. This site explains and proves the benefits using "Plain English"

Super Math Tricks The wonderful world of applied mathematics is full of great tricks you can use to perform complex calculations in your head.

Vedic Math - Is  a compilation of arithmetic shortcuts believed to have been written by ancient Indians during a glorious period in Indian history called the Vedic Age. It  is a unique system of calculations based on simple rules and principles, with which any mathematical problem - be it arithmetic, algebra, geometry or trigonometry - can be solved in one's head. It has now crawled into the 21st century as quick problem-solving ability becomes the most crucial link between aspiration and fortune for young Indian students.  
Check out this tutorial