XBMC ‘Atlantis’ Beta 1 released
Media-Portal is not the only open source HTPC program on the block, XBMC, perhaps known for their great looking skins, is another major contender in the arena. XBMC has evolved into a multi-platform software that supports Windows, Linux and Mac. Do keep in mind XBMC supports a lot of stuff but if you are looking for TV tuning you will want to look elesewhere.
If you want to get an idea on how pretty you can make XBMC, I encourage you to take a look at the graphics & skins forum located here. All I can say is WOW.