MythTV is an Open Source HTPC application that is ported to many Operating Systems including, LINUX, Apple OSX, and Windows. It feature many capabilities, however, it is chiefly used for Digital Video Recording. The commercial skipping feature is noteworthy.

Although it is primarily supported on Linux, it is compatible with other viewing systems such as XBMC (via Mythbox app) on an AppleTV.

Literally everyone, ever. Including the writer of this definition.

a shitty linux pvr for people who can't buy tivo or get one from comcast