AskOxford.com
“This site is for "questions on English grammar, spelling, and usage; to search a dictionary of words, names or quotations; for help with writing; for crosswords and puzzles; and to learn about language."
Elements of Style
“Asserting that one must first know the rules to break them, this classic reference book is a must-have for any student and conscientious writer. Intended for use in which the practice of composition is combined with the study of literature, it gives in brief space the principal requirements of plain English style and concentrates attention on the rules of usage and principles of composition most commonly violated.”
English Grammar FAQ
“Questions and answers about English usage and grammar from a linguistics professor.”
Grammar and Style Notes
"These notes are a miscellany of grammatical rules and explanations, comments on style, and suggestions on usage I put together for my classes. The entries here are of two types: specific articles on usage, and more general articles on style. The specific articles can be further divided into two classes: (1) grammatical rules and explanations, matters rather of precedent than of taste; and (2) more subjective suggestions for making your writing clearer, more forceful, and more graceful. The specific articles are intended for quick reference, such as when you have to find out whether which or that is appropriate. The general articles lend themselves to browsing and absorbing over time." Entries are browsable alphabetically. Includes a short bibliography.”
Guide to Grammar and Writing
“There's a lot more than grammar help at this site. Learn how to write effective paragraphs and essays in addition to finding out about all things grammatical. You can even submit a question about English grammar or usage. Lots of resources available, too, including an online dictionary and famous quotations about writing.”