XBMC

The mother of all mediacenters. Yepp, it's true (at least in my opinion). It's simple, looks totally awesome and plays just about everything (including rared stuff). I also have the Aeon Stark theme which is just ridiculously cool looking :D

XBMC can be found here and Aeon here.

Oh, I'm using IMDb's scraper for movies and theTVDB's for tv shows if someone might wonder.. :)