OSU Writing Center Online contains handouts on many writing problems and links to online writing resources.
The Purdue OWL (Online Writing Lab) offers an excellent set of pages about the writing process, with topics ranging from "Starting the Writing Process" and "Writer's Block" to "Writing with Feedback."
YourDictionary, the best dictionary site, covering many languages.
The Oxford English Dictionary (via the OSU library, login required)
Information about citation formats:
Information from the Purdue OWL about how to use citation generators responsibly.
Citation generators:
The Purdue Online Writing Lab (OWL) has handouts and materials on:
Voice of the Shuttle, by Alan Liu et al. at UC Santa Barbara, is a comprehensive website for humanities research.
Creative Commons is a resource for licensing and sharing creative work.
The Writer's Handbook, University of Wisconsin, with many student helps including an Editing Checklist explaining common errors and a Proofreading Handout.
Punctuation Made Simple, by Gary Olson.
Additional Resources on English Usage, Style & Composition.