it doesn't fit the original requirement as it is too large but when these discussions come up i feel obligated to mention silicon dust as well. http://www.silicondust.com/hdhomerun/

they make outboard tv tuners that will either do over-the-air or digital cable and stream wired or wirelessly. one model even transcodes to h.264 and they publish plugins for kodi and plex along with working with a variety of commercial and open source apps. it was my preferring tuning device with both myth tv on linux and Microsoft media center until i ditched cable entirely.