The Participatory Culture Foundation works on the development of Miro, which is a free open-source desktop video application that is designed to make mass media more open and accessible for everyone. From the screenshots, it looks so seamless it’s almost too good to be true.
UPDATE [2011-03-25]: I now use Iomega ScreenPlay Director HD and MediaPortal instead.
UPDATE [2012-06-20]: I found SMPlayer to be a very nice, cross-platform and lightweight alternative to both Miro and the popular VLC media player.