Purdue Online Writing Lab: Includes general grammatical rules and errors, MLA and APA format guides, and writing process guides.
Capital Community College Foundation: Interactive and comprehensive guides to sentence level, paragraph level, and essay level composition.
The Elements of Style: An online resource from the inimitable Strunk and White style guide.
Self-Study English Grammar Quizzes: Helpful online quizzes to sharpen or further your understanding of grammar. This website is especially helpful for non-native speakers.
University of Kansas Grammar Guru: Ask the grammar guru when you just can't find the answer anywhere else!
Excelsior College Online Writing Resources: Links to a variety of resources, including formatting and grammar guides, tips for evaluating sources, and essay writing strategies.
These are all available on the Apple App Store
Dictionary: Dictionary, Thesaurus, Rhyming dictionary, Spell checker, Vocabulary Building Games, and Wikipedia Broswer. This app has a library of words that can be accessed offline.
Mirriam-Webster Dictionary: HD version of the Mirriam-Webster dictionary
Related tip sheets:
The Journal: A list of creative writing prompts to get your mind going. This webpage has ideas for journals, free-writing, prose, poetry, memoirs, and opinion pieces.
Poets.org: From the Academy of American Poets. This database has poets, bios, selected poems (some with audio files!), essays, and more!
The Literature Network : Database of short stories, poems, quotations, quizzes, and trivia.
Creative Writing Prompts: Prompts for creative nonfiction, fiction, and poetry from Poets & Writers.
Fiction Craft Exercises: Compilation of several general fiction exercises to help develop your craft or work through writer's block from About.com.
Creative Writing Now: Prompts, how-to, resources, and contests.
Basic Sonnet Forms: Helpful website for learning about and composing Italian and Shakesperian sonnets.
Sestin-a-matic: An online sestina-maker that uses a form to help you compose your poem and takes the guess work out of the traditional poetic form.
Learning To Love You More: An art project and website filled with art and writing assignments to provoke memory, explore space, and define the story of self. The project officially stopped accepting submissions in 2009, but it is still maintained as an archive.