Ask HN: Interactive web graphics with RPC? (open source, of course)

I have an XML-RPC server written in Python. I would like to write a simple graphical web application. It is basically location-specific text rendering. The frontend should communicate with the RPC server to determine the appropriate responses to user mouse interaction.

What open-source framework/toolkit should I use? I am open to flex/html canvas/javascript/whatever, as long as it is open source, I can prototype rapidly, and I can communicate interaction with an RPC backend to determine updates.