This weekend I wrote a JavaScript version of my old Atari 800 game Dandy.
Check it out: Web Dandy
It was my first JavaScript application. It was about as easy as writing the Python version. I have only tested it in two browsers so far (Firefox 3.0 and Chrome), and only on one platform, OSX. I have already run into differences between the two browsers: Firefox 3.0 for OSX seems to double-post keydown events.
No sound or multiplayer yet. Oh, and I use the CANVAS tag, so I think older browsers (like IE 7) won’t work.