Grammar and Style Notes http://www.english.upenn.edu/~jlynch/Grammar/
Provides explanations of conventions of grammar, style, punctuation, and usage, broken into a file for each letter of the alphabet (for example: click on "a" for information on adjectives and adverbs).
The Lingua Center http://deil.lang.uiuc.edu/web.pages/grammarsafari.html
Helps you get beyond the "safe confines of grammar books" to explore English on the Web. Want to hunt for adjective clauses in Little Women? The Grammar Safari will show you how.
The Writer's Complex http://www.esc.edu/htmlpages/writer/workshop.htm
From Empire State College, this online writer's workshop offers good explanations and exercises. Click on Grammar Workout for advice on structuring sentences.
Ask A Linguist http://linguistlist.org/~ask-ling/index.html
Not sure whether to use who or whom? Go to this site, where a panel of linguists will answer questions about language.
How Can I Identify Weak Verbs? http://www.io.com/~eighner/qa050152.html
This essay by Lars Eighner suggests a hierarchy for distinguishing weak from strong verbs.
George Orwell's "Politics and the English Language" http://english-www.hss.cmu.edu/langs/politics-english-language.txt
Download this classic essay for Orwell's advice on adjectives, and also as a text well worth examining. Read closely to see if Orwell follows his own advice.
Wordnet http://www.cogsci.princeton.edu/~wn/
An online reference database of English nouns, verbs, adjectives, and adverbs that allows you to see words in many forms and to search for synonyms and antonyms (and much more).