The Purdue Online Writing Lab
Recently the OWL began adding slide shows, movies, and podcasts for students and teachers. The MLA and APA citation guides were already invaluable, but I've started to accept that students want content in digital form.
The podcasts' content focuses on rhetorical concepts. Because students struggle with ethos, pathos, logos, and kairos, any additional explanations are helpful. I'm for anything that helps students sort through the complicated textbook definitions of these concepts.
For a few years the OWL has been adding PowerPoint presentations on a range of writing topics. I'm not a fan of PowerPoint; slides are best used to introduce topics. Slides, by their nature, are superficial and should be accompanied by further reading and discussion. Still, the slides help students focus on key topics and concepts they should remember. The long list of available presentations is impressive and I do encourage teachers and students to browse the OWL library.
The OWL movies focus on visual rhetoric, but they too can be useful for writers. Unless we're discussing audiobooks, most of our words appear on pages or screens. Design affects the perceptions of texts, including how seriously a reader approaches the words. The OWL movies are a good starting point for discussions of visual rhetoric.
For the basics of academic writing, you won't find many resources equal to the OWL. Again, the website is: http://owl.english.purdue.edu/owl/