Javascript
NODE.JS IS GENUINELY EXCITING!
Yes, that's a signal boost.
I've only played with Node.js for about 24 hours now, and I'm already deeply impressed with it. Node.js is something of a holy grail: an implementation of server-side (and desktop) Javascript with a modern engine (Google's V8), in which all I/O is event-handled. You no longer care about multiplexing, spinning off threads, or any of the myriad gazillion other cares that server developers used to worry about. Instead, that's been built into a reactor core inside Node.js, and instead you receive notice of events (just like in browser-based Javascript), where the events will things like "header received," "body received," "message end," "connection made," and to which you'll be able to attach enclosured functions that will respond appropriately. It's an application server's base language on acid, and while I'm bad at making predictions, I suspect Node.js will be with us for a while.
Already there's a NoSQL database interface, the start of an application server, the start of a CLib-like library (like Python or Perl's standard library), and even a comet server. This could be fun.