I was surprised to see a lack of strain/torque gauges like those sold on digikey (http://www.digikey.com/product-detail/e ... ND/2503700
) on Sparkfun. Admittedly, I'm a mechanical engineer and only a hobby level electrical tinkerer, so I'm not immediately aware of the tech challenges with designing these.
but none the less, there are really awesome applications for these and I would imagine there's a large market for sensors in the sub-$100 range (if it can be done!)
I guess most of (if not all) of these require amplifiers, but if the sensors themselves could be made cheaply, I think it would be popular.