I was introduced to this game at the NSTA conference in Boston a couple of years ago. It is an awesome way to get your students to develop an understanding of the way nitrogen moves and changes forms.
Plus it is completely free. All you need is some card stock to print it on and some cute stamps or scented markers work well for students to track where they have been as they become a molecule of Nitrogen and move throughout the room from "station" to "station".
The Nitrogen Cycle Game