The Focus Pyramid is a lens calibration tool that assists with the autofocus microadjustment feature in newer cameras. It's made out of sturdy cardboard and is pre-creased so that folding is easy.
I've done microadjustments in the past by laying a ruler on the ground, having the camera on a tripod at a 45 degree angle to the ruler, and shooting test photos. I found the Focus Pyramid to be more effective because I felt I was able to control the test more completely. If you want an easy to use lens calibration tool, the Focus Pyramid is a good choice.