This site is full of "writing units to teach 21st century skills." The lessons are geared towards 9th graders, but can be used for high schoolers or possibly even college students. There are modules on how to write film/restaurant reviews and how to write a news story from "digital footprints." The idea is that journalistic-style writing is best for students because it allows them to write about things that matter and it is more suitable (than the five-paragraph essay) for online publishing.
Apple employee Mike Matas demonstrates the first truly interactive book on the iPad, at this TEDx talk. The highlight of the presentation was when Matas blew onto the screen, and this made a windmill in the book start to spin. This was an interesting look at how we might read books in the near future.
This book was a collaboration between several authors from the National Writing Project. It would probably be helpful for Language Arts teachers who find it challenging to assess online writing because it often fails to follow traditional writing conventions. In addition to addressing how to improve student writing, the book also contains chapters on Standards and Assessment for Digital Writing and Professional Development for Digital Writing.
In this book, education professor Troy Hicks guides teachers through the process of converting a traditional writer's workshop to a digital writer's workshop. Hicks discusses how we can use digital tools such as blogs, wikis, and multimedia compositions to help students become better writers. This book received several rave reviews on Amazon.
This assignment is geared towards college students, but could be easily adapted for high schoolers. Find a few long-winded blog posts written in a print style. Tell your students to rewrite the post in a more online-friendly way. They should identify main topics in the post, and then summarize each one. They could also try breaking up long paragraphs, using lists. This would be a challenging, but very useful, writing assignment for high school juniors and seniors, perhaps working in pairs.
This article from a freelance writer quickly covers the six ways. The one that hadn't occurred to me was accountability. This would be a great discussion topic and learning opportunity for your students. Through blog comments and forum posts, it's very easy for your readers to "call you out" over incorrect information or a simple disagreement. Obviously, this isn't desirable, but you don't want to stifle your voice for fear of consequences, either.