Chapters of a peer-reviewed multi-volume writing textbook, available for free download. Essays are diverse, and focus on the "why," not just the "how" of writing. Some example chapters are "What is Academic Writing?", "Reflective Writing and the Revision Process," "Navigating Genres," and "I Need You to Say “I”: Why First Person is Important in College Writing."
Writing help for students - includes information on different types of writing, steps in writing processes, citation styles. Also includes some activities for teachers to use, including Grammar Exercises, Punctuation Exercises, Spelling Exercises, Sentence Structure, Sentence Style, Eliminating Wordiness, Paraphrasing, Writing Numbers
Writing Across the Curriculum Clearinghouse
Writing Spaces: Open Textbook
Purdue Owl