A cheap flex sensor. It is not very precise, but might be useful for some projects.

http://www.youtube.com/watch?v=6lt6fb9za_A&feature=youtu.be

Parts:

1. A piece of PVC tubing 3.5 mm diameter and about 40 mm long
2. Two M4 nuts
3. Two M3x30 screws
4. A graphite pencil

Instructions:
Break the pencil, extract its core and grind it to thin graphite powder with a file. Put M4 nuts at both ends of the PVC tubing. Put an M3 screw from one end, it should fit tightly. Pour graphite powder into the tubing from its other end. Put second screw into place and tighten it. Connect your multimeter and measure sensor resistance. The resistance will probably be around 50 ohms in unbend state. To change this base resistance just tighten or loosen the screws a bit.