I love Love Letter, the popular card game by Seiji Kanai. It still amazes me how a game with just 16 cards can be so entertaining and exciting.
It sold millions of copies, is available in 28 languages, has 30 different official versions (at least one is published every year) and dozens of fan-made re-themes. It started the whole microgame craze which made lots of people design very small games.
To celebrate its 10th birthday this year I have made two videos.