Grammar Resources
Grammar Girl (short, friendly tips to improve your writing)
Grammar Bytes (information about grammar, writing, and presentations)

Usage and Reference Tools
Common Errors in English (an excellent site devoted to common usage errors)
The Elements of Style (The classic reference book for the conscientious writer) (Dictionary, Thesaurus, etc.)
The Internet Dictionary Project (includes foreign language translation dictionaries)
Bartleby Reference Resources (an invaluable list of links to online reference tools)
Internet Public Library
The Owl at Purdue (The famous online writing lab at Purdue)

MLA Format (Modern Language Association)
APA Format (American Psychological Association)

Project Gutenberg (a collection of over 100,000 titles)
Bibliomania (more than 2000 free classic texts)
Classic Authors (a collection of etexts categorized by authors' names)
Luminarium (anthology of English literature from medieval through the Restoration)
PAL: Perspectives in American Literature (an ongoing reference and research guide)
American Literature on the Web (a large collection of links)

NEW! Historical Guide to American Literature

NEW! Classic British and American Essays and Speeches (Over 300 to choose from)

Other Resources For Writers
Writing in various disciplines
Dave's ESL Cafe
English as a Second Language
Paradigm Online Writing Assistant

LifeHack Articles

10 Best Tools for Teachers and Students

Useful Online Resources for Writers

6 Tools to help Save Time Writing

10 Incredible Things You Learn From Writing 

Critical Thinking Skills 

The Critical Thinking Community 

6 Critical Thinking Skills 

What is Critical Thinking?

Excellent YouTube Video

No, You're Not Entitled to Your Opinion