I first learned about this website during a professional development day last year. I thought it was a great idea because it allows our students of today to use the technology they know how to use, without separate instruction.
Try voting on this question and watch the choices change:
Another website that I discoverd this spring was Prezi. This website is also free, and I really like this one because it allows for a teacher to create a presentation unlike any power point presentation. Students get bored with this type of presentation, so it can help motivate the students for class discussions, or more. Look over this example that I created for my classroom: