I know, I know, that's called a "lecture" and it's easy to do. But do you really have every student's attention? How about giving every student feedback on essays they wrote -- detailed, personalized, meaningful feedback and assessment -- all at once? Not only that, but the students can replay your comments and suggestions over and over, even let peers/parents listen and read to provide even more support and feedback. No more misunderstanding or "Uh, I forgot" what you said!
It all has to do with Google docs and screen savers, of which the details are all here, including great examples from actual student work. I think it's a great idea/tool, especially with middle school and up.
Props and thanks to The Transparent Teacher -- awesome blog!