Mythbuntu is an unofficial Ubuntu derivative focused upon setting up a standalone MythTV based PVR system.
Questions related to MythTV on Ubuntu or Mythbuntu should contain this tag.
More information on MythTV can be found here
More information on Mythbuntu can be found here
Other topics that may want to include this tag could be anything multimedia related that has Myth* (eg. MythVideo, MythMusic) in it.