There is certainly a great amount of hype surrounding HTML5, and even while the standard itself is years away from completion, some of its juiciest bits have already started leaking into browsers.
Besides the video and audio tags, one of the most prominent component of the HTML5 specification would have to be the canvas tag. This HTML5 element places within your browser a blank area which allows manipulation of its pixels by client-side scripts. Using a canvas in an HTML5 document, Javascrript can draw bitmaps which are rendered on the client side.
This little emulator allows you to load up a ROM from a list of games, and after a short period of downloading, play it right in your browser; no plugins required.
This is a brilliant look into the shape of the web in the future, where browsers will be able to run complicated applications without the need of any plugins, using just web-standard technologies.
Despite the obvious disadvantages of playing a game with clunky controls, at slow speed in a small window, this demonstration is significant, as it demonstrates what the browser is capable of even on its own.
Check out the project in action at http://benfirshman.com/projects/jsnes/
The author of JSNES highly recommends using Chrome, as it runs the code significantly faster.