Google is dropping support for H.264 in its Chrome browser, preferring to put more effort in free codecs like WebM, as Google sees that H.264 is licenced in a way that stifles innovation.
YouTube (owned by Google) is in the process of converting its contents to use WebM. If you want to access it now, you can join the testing phase. Firefox 4, due out in February, will fully support WebM as part of its HTML5 upgrade. There are a number of browsers already supporting WebM, and Google is planning to add WebM functionality to Safari and IE9. Note that Android, the operating system widely used in mobile phones, supports WebM from version 2.3 (Gingerbread) onwards - so Android 3.0 (Honeycomb) destined for tablets will also provide WebM functionality.
See also my more recent posting about for how the codecs vary in their 'openness'.
Problems with H.264/AVC/MPEG-4
While the MPEG LA announced that H.264 encoded internet video is free for end users to watch, people still have to pay large fees to create software that either encodes or decodes using the H.264 family of codecs. So the H.264 is not appropriate to be used in software under the General Public Licence (GPLv2 & GPLv3 – note that the Linux kernel uses the GPLv2 Licence), nor is it suitable for any software developed by people with limited budgets.
Mainly that WebM is a high-quality, open video format for the web that is freely available to everyone – not just to use, but also to people writing software that either encodes or decodes using WebM codecs. So the WebM codec is fully compatible with both the spirit and technicalities of open source software licensing. Soon lots of hardware will support WebM natively, which will further reduce the initial advantages of H.264.