Credo Reference Search the full-text of hundreds of reference books in the arts, humanities, social sciences and sciences. This tool seaches the text of the entries, allows for sorting of entries by subject and title. Also, use the Concept Map feature to explore related terms and discover disciplanary overlaps.
Blue Book of Grammar and Punctuation
Provides commonsense guidance on grammar and punctuation. Subchapters on common problems: e.g., subject/verb agreement, hyphens, apostrophes, etc.
The Elements of Style
Based on William Strunk, Jr.'s 1998 reference book, this site covers rules of usage, principles of composition, and other helpful writing tips.
Guide to Grammar & Writing Advice on grammar, punctuation, and usage questions arranged in drop-down menus under six headings: e.g., "Word & Sentence Level," "Paragraph Level," "Essay & Research Paper Level," etc.
The Text Doctor Writing-related reference sources in such sections as "Dictionaries/Thesauri/Vocabulary," "Grammar/Mechanics," "Resources for Technical Communicators," "Web Design Resources," etc.