Dictionary.com Grammar, Usage and Style

• Style Guide
• The Blue Book of Grammar and Punctuation - Jane Straus' reference guide and workbook
• The Chicago Manual of Style - A publication of the University of Chicago Press
• Common Errors in English - By Paul Brians. Simple, entertaining explanations of common errors in English
• English-Zone.Com - the BEST English-Learner's site on the 'Net!
• Guide to Grammar and Style - By Jack Lynch
• Sharp Points - Bill Walsh's "vociferous rants" on matters of style
• Silva Rhetoricae: The Forest of Rhetoric - A guide to the terms of classical and renaissance rhetoric. By Dr. Gideon Burton of Brigham Young University
• The Vocabula Review - A society is generally as lax as its language.

From the Open Directory Project:

• Reference/Style Guides
• Writers' Resources