mythtv
moovida ( formely Elisa) Media Center
Elisa is an Open Source cross-platform (Linux/Windows/Mac) media center connecting the Internet to an all-in-one media player.
Elisa Homepage
How-to install Elisa
Elisa Download (Strongly recommended you install from PPA
XBMC Media Center
Originally developed for Xbox game console under the name Xbox Media Center, the XBMC team has ported their popular software to various platforms, including Linux, OSX and Windows OS. Though not as friendly as Elisa, XBMC offers a full feature set and customization options advance users seek.
XBMC Homepage
How-to install XBMC
XBMC Download (Strongly recommended you install from PPA)
Aeon Project
Put some beauty back into XBMC, get Aeon!