Kodi (F.K.A. XBMC) is hands-down the final word in the media centre user experience. Before Roku and Apple TV, there was XBMC (the predecessor to Kodi). They’ve been doing it longer and–because it’s open-source–better than anyone else thanks to a bottom-up development infrastructure.
Kodi can handle all your media, in most any format, from any networked location and, thanks to its modular plugin structure, it can be extended to facilitate every home theatre contingency.
Installing Kodi in Windows or OSX is easy; simply download the installer binary package from the website and run.
Kodi in Linux is a little more complicated, requiring a little terminal work to get it started.
First, you have to install the required dependencies:
sudo apt-get install python-software-properties pkg-config
sudo apt-get install software-properties-common
Then, add the Kodi repository to your software sources and update:
sudo add-apt-repository ppa:team-xbmc/ppa
sudo apt-get update
Finally, install Kodi:
sudo apt-get install kodi
For more information, consult the Kodi wiki.
If you need to install an older version of Kodi/XBMC, check this article.