If you have Firefox Nightly build or Chrome you an play realtime gaming using WebRTC Data Channels.
WebRTC code example
Edit: I edited my post because after digging through the code, WebRTC doesn't quite look ready to go for multiplayer. I think it best to wait until Mozilla or somebody else makes a high level wrapper around WebRTC and that more browsers support it before we look into having an official plugin. I think we need to give it another 6 months. Still might be cool for someone brave enough to do something unofficial.