Purdue University's Online Writing Lab - this is a wonderful site full of useful information on how to site in MLA/APA and other styles. It also has sample outlines, workshops for resumes, and it is loaded with information. It is very easy to use.
Guide to Grammar and Writing - this is another very good site. If you are unsure what an adverb is, or want some information about formatting papers, look here.
Quoteland - helpful site to find quotes on numerous topics.
Citing Electronic Resources - this site is provided by the Internet Public Library - very helpful.
Strunk's Elements of Style - Classic reference book for English usage.
These are just a few of the sites found online; however, you must be careful to use sites that have valid information. Very good sites will come from educational institutions.