MediaPortal (OpenSource MediaCenter for Windows)

Found it through digg

"MediaPortal turns your PC in a very advanced Multi-Media Center / HTPC.
It allows you to listen to your favorite music & radio, watch your video's and DVD's, view, schedule and record live TV and much more. You get Media Portal for free/nothing/nada/nopes and best of all it is opensource. This means anyone can help developing Media Portal or tweak it for their own needs! "

What is more interesting to me is that it is built on .NET Framework. Cool

http://mediaportal.sourceforge.net/