Adafruit Pressure-Sensitive Conductive Sheet (Velostat/Linqstat) [ADA1361]

Specifications:

This conductive material (also known as "Velostat" or "Linqstat") is a nice addition for your wearable/sensor hacking toolkit. It's pressure-sensitive: squeezing it will reduce the resistance, so it's handy for making flexible sensors. And it's a lot less expensive than off-the-shelf pressure or bend sensors too! Each order comes with a 11"x11" (28cm x 28cm) 4mil (0.1mm) thick piece. We may fold the sheet into quarters to ship it to you. To remove any hard creases, simply lay flat on a table and put a heavy book on top to flatten it out.

This presentation from Hannah Perner-Wilson (plusea) has a lot of great ideas for how to use this material .