This page has a counterpart, Theora, for Ogg Theora video. Many of the techniques on Theora can be used to also encode WebM.
ffmpeg
ffmpeg -i input.avi \ -vpre libvpx-720p \ -quality realtime \ # best, good, or realtime; can also use qmin/qmax -threads 3 \ -acodec libvorbis -ab 128k -ar 44100 \ -f webm -y output.webm
See:
MEncoder
mencoder -ovc lavc -oac lavc -of lavf -lavfopts format=webm -lavcopts threads=4:acodec=vorbis:vcodec=libvpx -ffourcc VP80 input.avi -o output.webm
Confirmed working with r31918. More information available with this mailing list thread.
