This is the premier on-line writing center with all manner of useful tools for students and faculty alike including tutorials on developing, organizing, drafting and revising many kinds of papers. There are also handouts and tutorials on basic grammar, usage and punctuation.
This is a good faculty site with topics of interest to a variety of disciplines, primarily in the humanities and social sciences. There are links to "call for papers" sites, free software, universities, libraries, and journals as well as topics of general academic interest like pedagogy, job lists, union issues, and The Chronicle of Higher Education.
This is a good site for finding exercises, writing process strategies, and patterns for discovering, organizing, revising, and editing informal essays, thesis/support essays, exploratory essays, argumentative essays and documented essays. There is also a conversation board for posting and receiving advice on writing problems. There are also handouts of guidelines for over a dozen kinds of writing, from abstracts to critiques, to lab reports.
Developed for UVic's comp program, this site has gone "web-wide." This is the best college writers' site I've found. There are paradigms for six basic rhetorical modes plus sections on topics like introductory paragraph strategies and basic punctuation. All have many examples, and the research essay contains the best set of examples of integrating quotes into sentences that I've seen.