The Gestalt website says it better than I could…
“Gestalt.js is a library released by MIX Online Labs that allows you to write Ruby, Python & XAML code in your (X)HTML pages. It enables you to build richer and more powerful web applications by marrying the benefits of expressive languages, modern compilers, AJAX & RIAs with the write » save » refresh development model of the web.”
I haven’t had a chance to play with it at all, but the samples are pretty cool.