Liquid War 6 is an original multiplayer wargame. You have to control an army formed with thousands of pixels that look like liquid, and try and eat your opponent. This is a total rewrite of the (now quite old) Liquid War 5 version. It's still in beta stage, with many imperfections and no network play but it's playable now.
ArduinoPulseGenerator is a simple program for generating pulse sequences using an Arduino. There is an associated GUI that runs on the local computer, or you can simply connect to the Arduino with a serial console (9600 baud) and send it commands. This code has been tested on the ArduinoMega 2560 (timing accuracy ~ ±200 μs) and Arduino Due (timing accuracy ~ ±35 μs); it may work on other Arduino boards.
QMentat helps you practice mental arithmetic. It uses arbitrary-length fixed point arithmetic, and can handle any size numbers, only limited by the size of your screen (and in the case of division with an integer result, the speed at which the number can be factored). It is also quite configurable, allowing you to tailor the questions to your needs.