LeapTouch is a OSX application, soon to be available in other platforms, that can turn your monitor - any monitor* - in a touchscreen monitor. You just need a Leap hardware and that's it!
Once you download LeapTouch app, you just need to open it and an icon will appear in your status bar. There's a calibration option - follow the instructions on the screen, place your finger on your monitor borders when asked - that sets the app to use right away.
There's also a quit option, for obvious reasons. ;-)
Better support to multiple touch and a Windows version