Windows only: Media Center Studio allows you to create your own custom themes for Windows Media Center, allowing you to create a cooler, more customised WMC look more akin to what you'd expect from popular media centre, XBMC.
Photo from How-To Geek.
The application allows users to create themes using their own personal images. Currently it seems to only support PNG images for custom backgrounds — although it's easy enough to convert other image formats into PNG using other methods or tools (like recently mentioned Photo Magician). Media Center Studio has the same look and feel of many Microsoft programs, and even includes its own ribbon interface. While it can be a little confusing to figure out how to create your own theme, How-To Geek has a clear guide on how to create and apply one to Windows Media Center.
Media Center Studio is a free download for Windows only.