Citations:

• Sources: Their Use and Acknowledgements (Dartmouth College)

Grammar:

• The American Heritage ® Book of English Usage
• William Safire’s Rules for Writers

Spelling and word usage:

• YourDictionary.com
• Thesaurus.com
• Merriam-Webster Online

Business-specific:

• SEC Plain English Handbook: How to Create Clear SEC Disclosure Documents

In addition to online resources, The Elements of Style by William Strunk, Jr. and E.B. White is an excellent and inexpensive writing guide.
Available in paperback format, it retails for approximately $7.