I'm so incredibly jealous. Lady Ada over at AdaFruit Industries has all these great toys to play and experiment with, and she's figured out how to do it while enriching all of our hacker lives and making a little money to find more great stuff.
The 'toy' that triggered this post for me is some conductive rubber stretch cord that acts as a sensor. It's like being able to pull on the end of a resistor and have it's characteristics change linearly as it gets longer and shorter. Way cool! And it is incredibly cheap. She's priced it at less than ten dollars for a full meter and even includes a pair of alligator clips and a 10k resistor. Science teachers, for example, could dice it up and have enough for each student to have a piece for experiments.
The only drawback that I can see is that the sensor takes a little while to recover after being stretched, though I guess that could be compensated for in some applications by using two sensors in opposition.
As usual, the AdaFruit website has a great related tutorial page so you can learn while having fun.
(Via Conductive Rubber Cord Stretch Sensor + extras! ID: 519 - $9.95 : Adafruit Industries, Unique & fun DIY electronics and kits.)