We've linked to materials that might be useful for you in a variety of writing situations. There are sub-sections for a variety of specific topics linked on the left-hand side under "materials for students."
General Resources Online
The Purdue Online Writing Lab has a variety of handouts and PowerPoint presentations on writing. Their APA guidelines and MLA guidelines are particularly popular along with Chicago Style resources. They also have sections on grammar, mechanics, punctuation, visual rhetoric, the rhetorical situation, and avoiding plagiarism. Finally, there is a section on subject-specific writing, including literature, engineering, social sciences, creative writing, journalism, and professional/technical writing. Want to see everything they have at once? Click here for the site map.
The University Writing Center at Texas A&M University has a great database of materials, including analyzing scholarly articles, composing annotated bibliographies, selecting a topic, and writing a thesis statement. Many of their materials are presented as videos.
Writing Commons offers materials for writers grouped into four categories: process, research, genres, and community. Each section includes a detailed discussion of materials included (for example, the research section discusses textual research and empirical research) and links to supporting videos.