These Tools can be used in order to create WebM Files:
Sorenson Squish (Web based)
Sorenson Media is one of the first companies to offer a WebM compression Tool. You can try it here for free.
Wildform Flix (Win)
Wildform is one of the first to offer a free downloadable encoder fro WebM: Flix
Unfortunately it seems that they were also one of the first to take their free encoder offline.
Miro Video Converter (Win + Mac)
A free tool to encode various Video Formats including WebM/VP8: Miro Video Converter
The latest Version of transmageddon (0.16) now also creates WebM-Files.
Download it here.