I've had a good time playing around with LEDs. I've even made up a couple of experimental PCBs. And now I've dumped it all to the EverLED project's GitHub.
The PCB has an inline ISP programming "port" that I use sometimes: Reset, GND, VCC, SCK, MISO, MOSI from top to bottom. It's easier to route into a one-sided design than the 2x3 standard configuration. You might want an adapter, or to change the PCB yourself. :)
If not, there's a PDF suitable for laser-printer toner-transferring (it's mirrored already) and there's a .hex file suitable for flashing.
Thumb and fingers for scale.
The ultra-bright red LED shown here is pretty bright when looking directly at it, but almost invisible from the sides. I'm cheating here and pointing it at the camera -- it's a lot dimmer when it's diffused.
Needless to say, the 900 mAh LiIon battery show here is just for testing. In theory, that's 10 years' worth of dim LED. In practice, self-discharge and sheer boredom will dominate, and something smaller makes more sense.
What's next? With an application that only needs 10's of microamps, it makes power harvesting look interesting. I'll probably also make a small, finished version of the PCB, with a SMD ATtiny and everything compacted up. Or maybe a pushbutton...
But if you're interested in playing along, go to it! Everything in the GitHub is tested and works for me. Let me know if it doesn't work for you.