From the MAKE Flickr pool:

The Smart LED connects a single LED to a microcontroller and power source, allowing it to do fun stuff like relate the ambient temperature via pulses of light. Jim Blackhurst’s remix dispenses with the resistors for an even smaller profile, connecting the LED directly to the coin battery and ATTiny25 microcontroller, changing the RGB and flashing as instructed by the ATTiny’s built-in temperature sensor. See Jim’s Flickr set for more views.