A simple Canvas + WebSocket example built with jQuery and em-websocket. Watch a screencast here: http://screencast.com/t/ZmZjYWIx

To run yourself, fire up the EventMachine server in one tab:

./server.rb

And then start a simple HTTP server for the HTML page in another tab:

python -m SimpleHTTPServer

Then visit http://localhost:8000/ in Chrome or a WebKit nightly build with multiple windows and start drawing!