Imagine I have 100 LEDs. I want to have just one of them flash. Which one flashes is selected by the program at run-time. Of course I could easily do this if I had an Arduino with 100 i/o pins, but what is the best circuit-based way to do this if I have fewer pins available than LEDs (or if I don't want to buy an Arduino Mega to do the task that an ATtiny could do)?
Basically I would like some way to be able to select in the code which one flashes, and use fewer pins than the number of LEDs -- as few pins as possible per LED.