Which demos have made the largest impression on you as a developer?

One of the first that comes to mind for me is the demo of 280 Slides by @280north. It was a Keynote/Powerpoint app for the browser.

I remember being impressed because I hadn't seen very many apps of that quality built for the browser before. And 280 North was just 3 people!

Another demo that made a huge impression on me in my early years as a developer was the MochiKit.Animator demo released back in 2006 (not online anymore).

I was impressed at how MochiKit used functional programming, while most other js frameworks used a class-based model.