The Magnetic Linear Encoder v1.2 is a fantastic building block for positioning systems. The core of this board is the AS5306 chip. This chip is what is commonly called an encoder. What is does is measure and report its location. It does this by reading the changes in the magnetic field as the chip changes position. What this means is that when properly assembled, this board will allow you to detect the the position of something like a linear axis. If you combine this with a motor and a microcontroller, then you can build a closed-loop positioning system whose speed and position can be precisely determined and controlled, even if something unexpected happens.

Available in the MakerBot Store: http://store.makerbot.com/magnetic-linear-encoder-v1-2-kit.html

Docs at: http://wiki.makerbot.com/mle