1. The Effective Writing Marking Guide
The Department of English at the University of Calgary has prepared an alphabetical index of common writing errors with links to a series of excellent grammar tutorials. The explanations are straightforward, and the examples are clear and concise. Useful for teachers and students alike.
Our favorite OWL--the Purdue University Online Writing Lab--hosts this comprehensive collection of exercises.
This site (a companion to Andrea Lunsford's textbook The Everyday Writer) not only explains and illustrates the "most common surface student writing problems" but also provides exercises to help us correct those problems.
According to Big Dog's buddy, Doc Scott, this site offers "a list of the bare essentials of grammar. These are the things that English teachers love to comment on in your papers." Particularly helpful are the self-tests at the bottom of each page.
Convinced that language should be enjoyed, college instructor Robin L. Simmons has created the most delightful grammar site on the Web. "The goal of the site," she says, "is to cover each type of sentence error in real depth."