I'm sure by now you've all seen the wooden growth chart rulers either on Pinterest or from Pottery Barn Kids .
PBK doesn't sell this one anymore:
All you need is a wood board about 6 feet long, something to measure with, a straight edge, some stain or paint and a paint pen or sharpie marker.
Here is the one that I made back in November 2011 for my home. I have it hanging in my dining room near our kitchen.
Here is one that I made in May 2012 for a good friend of mine.