The Purdue OWL (Online Writing Laboratory) is a huge reference to improve your writing.  They have a large section of grammar and punctuation rules for native speakers for your most difficult grammar questions. About.com has a lot of resources although it may be difficult to find what you are looking for if you don’t know the correct terminology.  Use your textbook or another grammar reference to identify what you are looking for and then...

English Page is a good collection of fundamental grammar rules with explanations, examples and exercises.  A good first reference point.