Guide to Grammar and Writing: Here's all the information you need on all aspects of writing (comprehensive instruction on sentence level, paragraph level and essay level concerns). Much of the information can be downloaded in the form of a Power Point presentation. Also includes samples of business letters, application letters, resumes and research papers. The site is completely searchable, and as if that weren't enough, an amazing backlog of grammar questions and answers is posted. (If you can't find what you need, the site offers a personal answer to any grammar question you e-mail. Response promised within 1-2 days.
Purdue Online Writing Lab: This online writing center at Purdue University has an amazing library of 100 handouts on common writing problems.
The University of Victoria's Hypertext Writing Guide: Gives short lessons on writing-related topics with clear introductions to each topic area. Contains a dictionary of basic literary terms and common grading abbreviations (such as "AWK"). Please note that this site has been moved so the information/layout might have changed.
The Writer's Web: Created by the writing center at the University of Richmond, this site offers good "getting started" questions as well as articles on writing in different disciplines.
Bartleby.com / Great Books Online: Bartelby.com offers many literature classics online (in their entirety), for free. This site also offers on-line editions of dictionaries, thesauri, encyclopedias, quotations and style guides by such language mavens as Strunk, Mencken and Sapir.
APA Style Blog: The APA Style Blog is a companion to the official APA manual. The blog covers a number of tricky APA style and citation questions that may not fall neatly into a category covered in the APA manual, and the blog is also written in clear, easy-to-understand language.