H.264 (mp4) support in Flash
The third beta of the Flash player on Adobe Labs will support H.264 video, with full support in the "proper" version of the plug-in coming in the 4th quarter of this year.
Big news, and a big step towards taking on Silverlight. Lots of detailed insider information on this announcement can be found on kaourantin.net, with other sites I've read this morning including Yahoo!, Aral Balkan, and Flashmagazine.