Penguin Brief Handbook is an excellent source for grammar explanations and exercises for both native and non-native English speakers.
Writing Guides:
Visit Re: Writing for grammar assistance, to learn how to properly cite sources, for hints on avoiding plagiarism, and more. This one website can help you with all stages of the writing process, from thesis statement to bibliography.
Writer's Handbook is the University of Wisconsin's writing website. It contains information on writing research papers, science essays, application materials, and analytical literary papers.
Diana Hacker.com is one of the best on-line sites for documenting sources in MLA, APA, CBE, and Chicago style. There are also good sample essays and grammar exercises available.
References:
Merrian-Webster's Dictionary This on-line dictionary site finds definitions for input words and contains a very effective online thesaurus.