Silverlight might be missing input boxes, but now it has a physics engine. As impractical as it seems for most web applications, I cannot resist the urge to try it.
Announcement post
CodePlex site. Yes, it's open source
Demonstrations, as shown above
Quick start for some code samples and downloadable video tutorial with more samples.
Disclaimer The opinions expressed herein are my own personal opinions and do not represent my employer's view in anyway.