VP8
From Jon's Wiki
Here's how to encode to a VP8/Vorbis WebM file:
ffmpeg -i SOME_VIDEO.mkv -threads 8 -f webm -vcodec libvpx \ -g 120 -level 216 -profile 0 -qmax 42 -qmin 10 -vb 2M \ -acodec libvorbis -aq 50 -ac 6 SOME_VIDEO.webm
Requires ffmpeg >= 0.60