VP8

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 1800K \
   -acodec libvorbis -aq 5 -ac 6 SOME_VIDEO.webm

Requires ffmpeg >= 0.60 and assumes your playback can handle 5.1 surround Vorbis using libvorbis >= 1.4

References