MIT's Media Lab is working on a system of information-sharing that can use light-based communication to remind you of an appointment or program an Arduino. The coin-sized Droplet is an ambient display that flashes green or red to indicate a certain status — in this case, the progression of time. It can also recognize flashes of light and turn them into data. That means that if you press it against an ordinary screen running the right software, the screen can tell it about an appointment and set a timer on the Droplet, as seen in the excellent Engadget video below. To change the time, you can hold it to a tablet or other touchscreen; the tablet will read data from the Droplet, then let you reset the timer by rotating it.
StackAR, a second technology, works in a similar way, but it's connected to an Arduino instead of a timer. When you press it to a touchscreen, it displays a map of pin information and some software controls, letting you program the Arduino from the screen and transfer the information with flashes of light. Both StackAR and Droplet are obviously proofs of concept, but they allow for simple communication in an extremely intuitive way that could be used across a variety of platforms.