A few of the videos have been coded with XVid. All the rest have been encoded with libavcodec "mpeg4" option using mp3lame as the audio codec.

If you're a Linux user, you're lucky. MPlayer contains everything you need and is completely free (pricewise and rightswise).
Just remember that if you decide to use a CVS version, you need to add libavcodec manually before compiling.

If you're a Windows user, installing DivX and XVid codecs should help. If you download and install "ffdshow", both should work.