Windows: Most of the time, your keyboard works as intended -- but occasionally you want to throw it out a window. Instead of defenestrating your keyboard, try diagnosing its problems with Switch Hitter.
This application allows you to detect problems like ghosting, where multiple keys pressed at the same time aren't recognised, or chatter, where pressing a key once is recognised as multiple key presses. Switch Hitter can also detect the keyboard layout you have selected and which character codes a particular keystroke combination is sending. This is very handy if you're trying to figure out whether your hardware is broken, or if there's something wrong with your settings.