Meet the MyoWare: a muscle sensor system that you can use to build all kinds of fun wearable devices, control robots, and play games. The MyoWare is Arduino compatible as well. It measures muscle activity through the electric potential of a muscle and outputs an analog signal representative of how hard the muscle is being flexed.
The MyoWare can be used to build bionic gloves, claws, and other fun gizmos. It has an on/off switch and LED indicators to show when it is on. The developers are releasing all their shields as an open source hardware. You can find out more about it on Kickstarter.