VP8 Encoder - ivfenc - With Avisynth Script input by Nic
http://nic.dnsalias.com/ivfenc.zip

Useful Links:

aoTuV B5.7 Vorbis Audio Encoder that can take an AVISynth script
http://nic.dnsalias.com/AVSVorbis.zip 

Mosu's mkvmerge that can mux vorbis + VP8 to WebM
http://www.bunkus.org/videotools/mkvtoolnix/win32/pre/mkvtoolnix-unicode-3.4.0-build20100527-255-setup.exe 

Latest DirectShow filters for playback
http://code.google.com/p/webm/downloads/detail?name=webmdshow-0.9.7.0-20100526.zip
(http://nic.dnsalias.com/WebMx64.zip for 64-bit versions of Directshow filters)

WebM Project
http://webmproject.org/

Doom 9 Thread
http://forum.doom9.org/showthread.php?t=154669&page=8

FFDShow is needed for WebM Vorbis Audio playback - The Xiph audio Directshow filter isn't currently supported

Encoding:

An Example AviSynth Script:
AVISource("YourFile.avi").ConvertToYV12()

Encoding to WebM can then be done via:

AVSVorbis -q -2 in.avs out.ogg
ivfenc --target-bitrate=100 in.avs out.ivf
mkvmerge --timecode-scale 1000000 --disable-lacing out.ivf out.ogg -o out.webm

Cheers,
-Nic

ps
if you have trouble with seeking, remove the source filter so that you rely on the splitter filter (which tends to work better).
i.e. regsvr32 /u "C:\Program Files\Common Files\WebM Project\webmdshow\webmsource.dll"