All national park entry fees are waived one weekend a year--and this is that weekend.
Enjoy a trip to one and take in the natural beauty.
P.S. There is one correction needed to the article above. While the article states that Yellowstone National Park is in Montana, over 90% of the park, including most of its main scenic wonders are in Wyoming. Small parts of the park lie in both Montana, and also Idaho.
[ Edit: Edited on 23-Apr-2014, at 08:38 by Calcruzer ]