Mark Fickett made some cool light up avatars to indicate when players log on and off of his Minecraft server.

When a player logs into my Minecraft server, the associated folded paper avatar lights up: blue-green as they log in, white while they’re online, red just after they sign off.

A Python script queries a Minecraft server for player status, then sends a list of online players to the Teensy. The Teensy runs an Arduino sketch that translates the list of names to output pin settings, controlling the LEDs.  You can find the code for the project on Github.