The Purdue Online Writing Lab
This is a great, easy to use website for you as a student and as a tutor. On the upper navigation you can choose "ESL" to get easy access to grammar & mechanics, practical exercises for practice and writing guides for American and international audiences. You also have access to general writing guidelines, multiple research and citation guidelines and job-specific writing strategies.
University of North Carolina at Chapel Hill: The Writing Center
This writing center has great information on writing styles, the writing process and writing for specific purposes. Their "Citations, Style and Sentence Level Concerns" provides great examples and explanations of common grammar, mechanics and usage errors.
Dartmouth Writing Program
Dartmouth has a short, but effective presentation of the "American" argument style. Many students from different cultures find American thesis and papers styles are difficult to understand. This website is a good resource for you to understand common problems.
Dave's ESL Cafe
Dave's ESL Cafe has great resources for tutors and tutees, everything from grammar, phrasal verbs, pronunciation, etc.