Other
Sharpening Your Writing Skills
This handbook provides step-by-step modules to help you develop an appropriate business writing style. It also gives you rules for punctuation and "plain English." Answers are provided for exercises. PDF document.
Online Writing Lab at Purdue University
Purdue University Owl: General Writing Resources
This helpful resource details the process of writing, including how to plan, write, and revise. Content also includes the different types or genres of writing, and exercises in grammar and mechanics.
Other
Northwest Regional Educational Laboratory: 6+1 Traits
This site focuses on the 6+1 TraitTM Writing framework.
BBC
Bbc Bitesize: Writing
The BBC offers quick (bitesize) activities, revision options, and quizzes on a variety of writing skills, covering writing arguments, writing factual documents, and writing stories.
Other
Brain dance.com: Three Step Creative Writing Process
This process seems very involved, but there are good ideas for getting started on your creative writing if you're stuck at the brainstorming stage.