Zen photon garden is a 2D ray tracer in your browser. A blog post gives examples of what's possible and explains some of the unique details of its implementation.
Play Dune II in your browser.
Heres one for the Opera (the browser not the singy thing) snobs out there.
If you've never lived in California, you really ought to give this site a try. We haven't had a big one in a few years, so I guess we're due for a devastating one any day now...