My friend Xavi which is a sports instructor and also a circus performer got interested in Linux and free software a few years ago. He decided to learn C#, Gtk# and create his first GUI application some time ago and has now shipped it.
His application is ChronoJump. Its interesting because he was the first user for the Gtk# port of NPlot.
He also built some custom hardware to take the measurements:
Some NPlot Graphs.
Web report generation.
More screenshots are here.