The SparkFun COM-08644 is a small square magnet.

These are very small rare earth magnets - 0.125" cubed. Composed of Neodymium/Iron/Boron (NdFeB), these magnets are curiously strong and make for a great way to attach tools to a metal workbench, hold small children to a fridge, and as a trigger for reed switches. Don't let the photo fool you - these arevery small magnets.

Can Activate a reed switch from a distance of about 1cm.