POV devices are quite popular because of their nature. With single line of LEDs you can display text messages, show animation and other stuff that fits in to matrix like display. Main disadvantage is that devise has to be waved in front of viewers eyes. But hey, you get it in size that fits in to pocket.
Following project is based on Attiny2313 microcontrolelr that controls 8 LEDs and is powered by two AA batteries. Actually board size is same as battery pack, so it quite compact size. Seems it is unfinished, but the goal is to build simple GUI for uploading text messages via USB to UART. Great project that would take couple hours to build.