Indispensible Writing Resources: An extensive guide to resources on the web and in print. An excellent place to start looking for materials.
Writer's Handbook: A guide for writers, produced by the UW-Madison Writing Lab
Two online Guides to Grammar:
Hypergrammar
Guide to Grammar and Writing
Online Guides for MLA and APA Documentation Style:
MLA Style Guide
APA Style Guide 1
APA Style Guide 2
Some useful general reference works:
Merriam Webster Online
Dictionary.com
ARTFL Project's Roget's Thesaurus
Encyperpedia