HTML5 video tag which will remove the need for Adobe's proprietary Flash to display videos online. Currently YouTube uses the H.264 codec to compress their videos which has a very good performance rating but is encumbered by patents. Ogg Theora video compression format on the other hand, is open source and free of proprietary licensing and copyrights, therefore free of royalty obligations. The Problem with Theora is that it does not match the performance of the H.264 codec.
Mozilla and Opera have decided that there is no question as to which codec should be the standard since the web is ultimately based on open standards. They have already implemented support for Ogg video and Google has quickly followed suit with their Chrome browser. Chrome however, also supports the H.264 codec putting Google on the fence with this dispute. Apple is loudly in support of H.264 but where is Microsoft in all this? They seem to oblivious to the dispute and Internet Explorer has no support for HTML5 at all. Nuanti, a company that develops web browsing technologies, has developed a plugin for Internet Explorer that will give users the ability to make use of the HTML5 video tag but is limited to Ogg Theora.
When we tally up the results we have three popular web browsers supporting Theora natively plus one through plugin support (Firefox, Opera, Chrome + Internet Explorer). This against two H.264 browsers (Chrome, Safari). The user-base is clearly in favour of the open codec as Internet Explorer and Firefox together hold more than 85% of market share. It's also worth mentioning that Google recently acquired On2 Technologies which developed the VP3 codec from which the Theora codec was derived. Perhaps this is a sign that Google will support Theora and is actively ensuring that it will meet the standards of H.264.
Ars Technica has a detailed article on the topic.