Paradigm Online Writing Assistant. A lot of information about how to write informal essays, thesis/support essays, argumentative essays, and exploratory essays.
ESL Bee. For high intermediate or advanced English learners who have never taken a formal English writing course.
Basic Paragraph Structure. Detailed discussion of the topic sentence, body sentences, and the concluding sentence.
The Purdue University Writing Lab Handouts. One of the earliest online writing lab. The Handouts section provides excellent guide to academic writing. There is also a section of English as a Second Language.
Writers' Workshop. Grammar handbook, citation styles, ESL resources, and writing tips, from the University of Illinois at Urbana-Champaign.