English Grammar Online 4U is an excellent writing resource that provides a lot of very useful tips without getting too complicated.  Their General Information on Writing English Texts is a very good place to start improving your writing.

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...