This web site sells a grammar and spell checker, but they also have a compendium of rules for free.
"Looking for a specific rule to help you in your writing? Trying to decide between two similar words? A grammar checker helps you, but does not tell you why. Grammar checkers also miss many errors, especially those having to do with names, punctuation, sounds, and style.
Some of you may have a full grammar textbook stored on your disk somewhere, but it is a nuisance to access and use.
Grammar Slammer takes care of both problems in an easy-to-use format. Grammar Slammer contains the rules and tips you need to write your best and make yourself clear. Grammar Slammer uses the familiar Web Page format to make it easy to find what you are looking for. It even has an easy-to-use glossary to help with those grammatical terms you can't remember. It will truly Slam your Grammar Agony!"