Spacewolff - P2P marketplace for ads. Turn excess space into ad space.
Postacard - Text a photo to send it as a postcard for $3.
StickerRide - Let your car earn money through advertising
Timelense - Bring postcards to life using AR (and a sprinkle of magic).
Grabb-It - Location-based ads on your Uber's window
Send Mail Get Mail - A postcard-sending website.