Grammar Links
The Purdue "OWL" contains grammar handouts, exercises, workshops, and links to other writing labs. This may be the most comprehensive online writing lab in the country.
This site, designed by Prof. Jack Lynch of Rutgers, is a good source for explanations of the conventions grammar and punctuation.
Designed by Prof. Charles Darling at Capital Community College in Hartford, this site has grammar information, quizzes, and PowerPoint presentations.
This site, developed by Gary Olson, provides an easily accessible review of punctuation.
Several years ago, Andrea Lunsford and Robert Connors compiled a list of the most common errors in student papers. This link to the website for the St. Martin's Handbook (5th edition) by Andrea Lunsford provides a list of the errors with the relevant rules and examples. While you will not have access to all the exercises unless you purchase the text, you might want to use the list as a means of self-assessment or review.
Reference Links
Merriam-Webster Online Dictionary
This site provides links to dictionaries in more than 200 languages. It also contains specialty dictionaries in fields such as medicine, business, finance, and computing.
Usage Resources
Paul Brians, Common Errors in English
American Heritage Book of English Usage
Hutchinson Dictionary of English Usage
Tameri Guide for Writers
Grammar Curmedgeon - Usage and Grammar
Garbl's Writing Center: Style and Usage