If you're looking to create some WebM videos to try out the new WebM support in Firefox 4 Beta 1, an easy tool to use is Miro Video Converter. Because WebM is such a new format, existing video editing software doesn't encode in WebM yet, so if you want to make a WebM video, you must create it in another format and then convert it to WebM using a tool like Miro Video Converter.
Miro Video Converter provides a very simple interface for ffmpeg, one of the leading open source video encoding tools. Miro Video Converter configures ffmpeg to output converted video to the same quality as the input, and it uses the excellent xiph.org libvorbis audio encoder, rather than ffmpeg's own broken Vorbis encoder (if you hear poor quality audio on YouTube WebM videos, it's probably because they're still using ffmpeg's vorbis encoder).
Miro Video Converter is freely available for download on Windows and Mac, and can convert to other formats besides WebM.