Epiphany web browser is probably the only browser which doesn't need an update to be able to display WebM videos.

You do need however an updated Gstreamer which Ubuntu users can install from the Gstreamer devs PPA:

Both Firefox and Opera already released WebM (VP8) capable nightly builds which you can download from here:

To run Opera in Ubuntu, download the archive, extract it and double click the "opera" file. Then try out this video which already uses WebM (with VP8) - but don't forget to enable HTML5 for YouTube before watching the video.

Ubuntu users: if you want a WebM capable Chromium, add the Chromium daily PPA - the latest builds (as of 20 May) already have WebM support. Install it with:

sudo add-apt-repository ppa:chromium-daily/ppa && sudo apt-get update
sudo apt-get install chromium-browser chromium-codecs-ffmpeg-extra

"chromium-codecs-ffmpeg-extra" is required by WebM it seems.

Note: Google Chrome (not Chromium) will be getting WebM support starting with May 24.

As you probably know, a Google I/O conference was held today and a lot of blogs said they will announce big things. And big it was: Google officially announced the release of an open source, royalty-free video format called WebM which will be using the VP8 codec Google aquired from On2 as well as Vorbis audio.