Here's a cool one for Media Center

Sourced from Hacking Windows7 MediaCenter: Download Theme: “Homeworld”

The new theme for Windows 7 Media Center based on the Homeworld is now available for download.

Download and install Homeworld.msi
Close Media Center and wait 45-60 seconds for the system files to be released
Run the Enable Homeworld Theme shortcut as administrator (right click and select “Run as Administrator”)
When the patch dialog appears press start. If you receive an error it is likely the system files haven’t been released yet, wait 60 seconds and try again.
Once the patch is applied successfully close the patch program
Restart Media Center

In the case of epic failure open Windows Explorer and go to “C:\Windows\ehome” select “ehres.dll” and rename it “ehres.dll.bak”. Find “ehres.dll.original” and rename it “ehres.dll”

This download is provided as a public beta release. It has been tested to work on both x86 and x64 Windows7 Beta1 Build 7000.
Looks like a good site on W7MC.