Adafruit Short Flex Sensor

This sensor can detect flexing or bending in one direction. They were popularized by being used in the Nintendo PowerGlove as a gaming interface.

These sensors are easy to use, they are basically resistors that change value based on how much their flexed. If they're unflexed, the resistance is about ~25KΩ. When flexed all the way the resistance rises to ~100KΩ. They're pretty similar to FSRs so following this tutorial will get you started. You can use an analog input on a micro-controller (with a pullup resistor) or a digital input with the use of a 0.1uF capacitor for RC timing.

The bottom part of the sensor (where the pins are crimped on) is very delicate so make sure to have strain relief - such as clamping or gluing that part so as not to rip out the contacts!