LimeJS is a HTML5 game framework for building fast, native-experience games for all modern touchscreens and desktop browsers. Its goal, according to the creators, is quite clear and simple: to provide an easy way to build good game experience without thinking about inner workings.
Check LimeJS‘s official site or its GitHub repository.
Another great example of some of the things that can be done using HTML 5 features: Dale Harvey created a playable HTML5 Pacman.
Wow! A group of googlers developed a HTML 5 based port of id Software‘s Quake II. So, we can play Quake II in a common web browser (well, just Safari and Chrome for now) without any plug-ins. You can read about the project on Google Web Toolkit Blog, watch a demo on YouTube or get the code on the project page on Google Code.