One-stop guides to all aspects of writing
Purdue OWL (Online Writing Lab)
The Purdue OWL has everything - LOTS of advice on academic writing, as well as details of the most common style guides (APA, MLA, Chicago).
George Mason University Quick Guides
Many, many links to practical writing advice for all genres and situations.
UNC Writing Center Tips & Tools
A comprehensive collection of academic writing resources.
CMU Academic Resource Center
Lots of videos and handouts about writing, including resources for English learners and links to other university writing centers.
Central European University Online Writing Resources
A fantastic bank of resources from our neighbors at the CEU Center for Academic Writing in Vienna.
Writing guides for specific fields
Duke University Science Writing Guide
A short guide to clear and concise writing in the sciences.
Organizing Your Social Sciences Research Paper
A very practical step-by-step approach to designing and writing a paper in the social sciences - but many of the tips are useful for ANY field!