In January, YouTube launched a player that used the HTML5 video tag. To try this player, you have to go to youtube.com/html5 and enable the player. If you have a browser that supports the video tag and the YouTube video doesn't use captions, annotations and doesn't show ads, you'll be able to watch the video in YouTube's HTML5 player.
What if you want to switch to the HTML5 player for a single video or you want to link to the HTML5 version? YouTube has a special parameter that lets you do that: just append "&html5=True" to the video's address. Here's an example:
For now, YouTube's HTML5 player works in Google Chrome (h.264; Chrome 6 adds support for WebM), Opera 10.6+ (WebM), Safari 4+ (h.264) and Firefox 4 (WebM).