I can't stop thinking about node.js.
If you still need more convincing, check out Ryan's slides from his JSConf.eu talk (PDF). They're very readable slides.
It's still early days for node.js. The project (what do you even call it - a server, I guess?) can serve up HTTP very well, but it doesn't give you a framework by itself. Consequently there are frameworks popping up all over for it, following all the patterns of frameworks implemented for other languages. One thing the server is genuinely missing is database support. A PostgreSQL binding is in the works, which sounds great to me. MySQL might be harder because it doesn't have a non-blocking library, but who needs MySQL when you have Postgres? :-P