Paradigm Online Writing
Assistant Professor Charles Guilford's site includes an excellent discussion of the writing process and pays particular attention to common writing errors. Guilford's tips on the discovery process, organizing, and revising will also benefit all writers.
Writer's Handbook
The University of Wisconsin Writing Center includes guidelines on sentence construction and development. Topics considered in the Writer's Handbook include academic writing, research, peer reviews, documentation styles, grammar and style guidelines, letters, and essays.
Purdue University's OWL
Covering planning, drafting, and revising, Purdue University's online tutorial includes many instructional handouts, advice, and quick-reference guidelines. Their examples and exercises are designed to help students develop effective writing skills.
One Last Look: The Final Quality Control Review
For more information on revising, read Robin A. Cormier's excellent essay.