Since I'm using Foreflight with a Stratus 2 ads-b in, I'm very interested in the announced transponder from Appareo Systems. It's supposed to be tray compatible with a Garmin 327 which is what I'm using now. It also provides a cable to both power the Stratus 2 and to provide it with the "ownship" signal to prevent false positives from your own plane.

It's vaporware at the moment, so we'll see.
http://www.appareo.com/aviation/ads-b/