It's not free cable, at least not in the sense that you know cable.
It's XBMC (recently renamed KODI), think of it as open source software to build your own ROKU box.
If you want to see what it's all about go here,
Kodi | Open Source Home Theatre Software download, install, run on any PC.
If you like it and want to build a dedicated box for it get a Raspberry PI.
If you have a good size SD card and cell phone USB charger you can buy the bare board for $25.00
Complete kit will run $50.00
What these companies are selling is essentially a low cost Android compatible board with KODI (XBMC) pre installed and a bunch of links to streaming content sources. Be forewarned many of those sources do not stream legal content. You see a lot of content ripped from Oscar screening DVD's.