There are many, many useful resources on writing... too many, in fact. I've focused here on sites that offer comprehensive advice for planning and structuring written work.

MIT Writing and Communication Center. Simple, straightforward advice about writing structure and process.

University of Wisconsin-Madison Writing Center. A nice collection of resources that describe how to plan and structure different types of writing assignments, how to avoid plagiarism, and how to write clearly and persuasively.

Paradigm Online Writing Assistant. This online writer's guide is structured as a series of brief tutorials on the different phases of writing and editing. Many of the sections include practice activities.

Writing@CSU. Loads of resources from the Writing Center at Colorado State University. Most useful are the Writing Guides, which cover a wide range of topics related to composition.

Princeton Writing Center. A useful collection of handouts on effective writing practices.

Purdue Online Writing Lab. Lots of common-sense information and guidance, including handouts and samples.

Harvard Writing Center. Though not in a very Web-friendly format, the General Guides on this site contain lots of good information - particularly those on structuring essays and on revising and editing drafts.

Online Technical Writing. Online textbook for technical writing. Includes lots of samples and examples of business, technical, and scientific writing.

Resources for Writers and Writing Instructors. A long list of links to online writing resources from Jack Lynch of Rutgers University.