I wrote a simple #FOSS Web app called Altimeter Practice. As the name implies, it's for practicing how to read altimeters. Most instruments used in #aviation are fairly simple to read (an airspeed indicator, for example, is not much different to read from a car's speedometer), but three-hand altimeters are more complicated. This site shows a random altitude from 0 to 18,000 feet, and you read the altitude and type it into the text box.

Thank you to Raymond Blaga, Edward Hanna, Pavlo Kuzhel, and Sébastien Matton for developing the original code that I use to display the altimeter.