Meet the EasyTouch: a capacitive touch sensor that turns anything into a touch button. Just alligator clip it to anything conductive, and you are set. As the below video shows, you can create amazing projects with the EasyTouch. The EasyTouch module can be attached to Arduino, Raspberry Pi, and other DIY kits. For your Arduino, use the EasyPlug sensor shield.
The EasyTouch is simple to use and quite a lot of fun. You can find out more about it on Kickstarter.