Http. Javascript on the Server. Running Javascript on the server seems to be gaining in popularity.
Last year I wrote a small framework to test writing web applications in Javascript. Tony Garnock-Jones later extended this to support building continuation based web applications. More recently, Peter Michaux has taken my original code and made a nicer web framework out of it. John Resig has written a browser DOM emulation layer that allows jQuery and some other frameworks to be run server side. His example code looks very neat.