XBMC - A FLOSS Media Centre
XBMC is a great media centre application. It has a large, active user base and is actively developed. It is a good alternative to MythTV for those who want to use a media library rather than recording TV shows. It can also work as a client to a MythTV backend.
They accept donations via Paypal.
Donation link: http://xbmc.org/contribute/donate/