Dictionaries, Thesauri, Etymology, and  General Reference

Answers.com
Dictionary.com
Latin Dictionary and Grammar Aid
Merriam-Webster
Online Etymology Dictionary
The Phrase Finder
Thesaurus.com
Wikipedia

Language Translation

Babel Fish
InterTran
WordReference.com

Style and Grammar

American Psychological Association
APA Style: A Guide
Common Errors in English
English Grammar for ESL Learners
Modern Language Association
Plain Language
The Chicago Manual of Style
The OWL At Purdue

Books and Literature

Amazon: Books
Bartleby.com
Barnes & Noble
Borders
EKU Literature Resource Center
gnooks
Half.com: Textbooks
Joseph-Beth
PennEnglish: Calls for Papers
PoetryFoundation.org
The Online Books Page

Literary Theory and Criticism

@SLE Online
The Johns Hopkins Guide