People call Shoshone Falls "The Niagara of the West." It is not as wide as Niagara but the water drops free for 212 feet (vs. 176 ft at Niagara). The big advantage over Niagara: It is a five mile ride on back country roads to Shoshone Falls Park, and when we were there on 4th of July, the park was totally deserted which made for an incredible experience.
Big drawback, however: due to massive irrigation, there is not much water flow during the summer and sometimes it falls almost dry. The pictures were taken on July 4, 2006 and the water level was reported as "medium."
Wikipedia has more information.