Writing Skills
Guide to Grammar and Writing
Not only is this site a handy guide for anyone who wants to improve their basic writing skills, it has an "Ask Grammar" feature for submitting questions, lots on online grammar resources, and, according to Choice, "multitudes of links and all links tested were active."
Text Doctor
According to Choice, this fast-loading site provides an "exceptional collection of writing-related reference sources" with links to a number of essential writing tools.
Writing User-Friendly Documents