A blazing-fast, lightweight ... just kidding haha. A text interpreter that helps you memorize chemical valences.
If you don't have Python 3.4 installed, just click Run here to run the program, typing your answers/commands in the simulated console on the right. (It should be black.)
Instructions: Type QUIT to quit, SKIP to skip and one or two space-separated numbers (without a + sign) to represent the valence(s) of an element.