MythVideo is a part of the mythplugins package. See MythPlugins for instructions on downloading the tarball.
MythVideo will allow you to use an external program to watch media files that are not directly supported by MythTV.
$ cd ~/mythplugins-0.21
$ ./configure --disable-all --enable-mythvideo
$ qmake mythplugins.pro
$ make
$ su
# make install
# exit
See MythVideo's README file for additional information.