GB-PVR
GB-PVR has a very large listing of open source plug-ins and skins, however the program itself isn't open source. You can make your GB-PVR look like a Star Trek console, the rainforest, Tivo, or Windows MCE using skins (an example skin is shown below). GB-PVR can accommodate POP3-based email, let you play gaming ROMs and check the weather and movie theatre listings. Basically, if you can think of it, most of the time GB-PVR has a free plug-in to be able to do it.

Alternative skin (image courtesy of GB-PVR)
To use a plug-in you copy it into a GB-PVR directory located in program files under devnz then enter the GB-PVR configuration menu and the required info for the plug-in. Some plug-ins also require extra files to be copied into the directory.
Plug-ins are what you will spend the majority of your time configuring; if you want the extra features you're going to have to do the work. They can be very frustrating if you forget to copy in the extra files that you need for certain features to work. Just remember it's better than having to try to learn Linux in order to run Myth TV. I'm not trying to put down the Myth TV community, but you will go through more just getting Myth TV running than you will configuring GB-PVR plug-ins.
Even so, once you get GB-PVR working with the plug-ins, you need save a copy of the directory to restore if you have to reinstall. I won't go into the details of GB-PVR's plug-ins here, but you'll find a summary of some of the more useful ones at the end of this article.