William Strunk's The Elements of Style (1918 edition, online): Despite its age, this short classic will do more to improve your writing style than any newer or longer book.
Online Resources for Writers: A good collection of links to many of the major online writing resources.
Writing Guide: A useful general guide to writing papers, from Henson College.
Guide to Grammar and Writing: Excellent and thorough discussion of grammar and composition, inludes tutorials, links, and FAQs.
Style Guide Resources: A detailed list of style guide resources for the MLA, APA, Chicago, and CSE styles.
Home Page of William Stallings