One of the interesting web APIs that has so far remained Chrome-only is the Web Audio API. It's a real-time audio API designed for games and apps that work with sound beyond just playing a song or a short loop.
There already are some very interesting things being built on top of it, but they're all mostly in the demo realm.
Also in the demo realm is Google's latest Chrome Experiment, the JAM with Chrome. But just because it's technically a demo doesn't mean it's not fun.
It's not just fun, it's impressive, it's an app that enables you to make music in your browser in real time using only standard web technologies and, this is the best part, along with your friends.
JAM with Chrome goes beyond the Web API, it uses WebSockets, HTML5 Canvas, CSS3 as well Google cloud services to make it all work.
You get 19 instruments to choose from, all the way from old school drum machines to electric guitars. When it all works, it's impressive to say the least. However, there's a chance things will go wrong, as this is all mostly experimental technology.