The video format struggle on the web continues, and the three contenders seem to be Ogg Theora, H.264 and WebM. Most video sites seem to use H.264, while others use Ogg Theora. Interestingly enough, the comments I see give me the feeling that a lot of people believe H.264 is the most supported video format in browsers, but according to the Wikipedia article on HTML5 video, this is not true. Check out the nice table of supprted formats in different browsers there. The format supported by most browsers is Ogg Theora, supported by released versions of Mozilla Firefox, Google Chrome, Chromium, Opera, Konqueror, Epiphany, Origyn Web Browser and BOLT browser, while not supported by Internet Explorer nor Safari. The runner up is WebM supported by released versions of Google Chrome Chromium Opera and Origyn Web Browser, and test versions of Mozilla Firefox. H.264 is supported by released versions of Safari, Origyn Web Browser and BOLT browser, and the test version of Internet Explorer. Those wanting Ogg Theora support in Internet Explorer and Safari can install plugins to get it.
To me, the simple conclusion from this is that to reach most users without any extra software installed, one uses Ogg Theora with the HTML5 video tag. Of course to reach all those without a browser handling HTML5, one need fallback mechanisms. In NUUG, we provide first fallback to a plugin capable of playing MPEG1 video, and those without such support we have a second fallback to the Cortado java applet playing Ogg Theora. This seem to work quite well, as can be seen in an example from last week.
The reason Ogg Theora is the most supported format, and H.264 is the least supported is simple. Implementing and using H.264 require royalty payment to MPEG-LA, and the terms of use from MPEG-LA are incompatible with free software licensing. If you believed H.264 was without royalties and license terms, check out "H.264 – Not The Kind Of Free That Matters" by Simon Phipps.
A incomplete list of sites providing video in Ogg Theora is available from the Xiph.org wiki, if you want to have a look. I'm not aware of a similar list for WebM nor H.264.
Update 2011-01-16 09:40: A question from Tollef on IRC made me realise that I failed to make it clear enough this text is about the <video> tag support in browsers and not the video support provided by external plugins like the Flash plugins.