Embedding and running Node.js within a Firefox XUL extension. Impressive what you can do with Firefox and its infrastructure. Theoretically, you could also modify this extension so that it runs in a single-site browser, which then directly opens the homepage of the embedded server, allowing you to build completely autonomous desktop applications in JavaScript (and for those wondering why I'm collecting all these JS-on-desktop things: with the help of Phonegap and some glue, you could not only build apps for iOS and Android, but also desktop applications with the same code).