A kick-ass series of games for the GBA that use sunlight to incinerate vampires and ghouls. There's a solar sensor built into the cartridge that gathers sunlight from the real world and turns it into energy in the game.
Boktai is sadly underrated and relatively unknown, even though it is the most awesome game ever. One reason why it's not so popular is because a lot of gamers are afraid of sunlight, which is a stupid reason to not buy the game. You can always buy a UV lamp, for goodness' sake. Losers.
However, if one goes to /v/ on 4chan and posts a picture from the game, almost everyone there will recognize it.
"Man, Boktai is such an awesome game! Let's have a Boktai thread, /v/!"
"BOKTAI, HECK YEAH!"