I have really liked the Chimani National Park Guides and now the long awaited Yellowstone Park guide is available in time for summer travel. Even better, the app is free until July 17, so if you're heading to one of our greatest National Parks start downloading.
The Chimani guide for iPhone includes detailed maps of the park, descriptions of the 30 most popular hiking trails, GPS support to show where you are and what is nearby, a 60 minute audio tour, along with audio descriptions of 50 points of interest on the Grand Loop. There are sunset and sunrise times, which is great for photographers, and you'll get current events and lecture times at the park. The information is searchable, and even push notifications if you want them about happenings at Yellowstone.
The Chimani guides have been uniformly excellent, and Yellowstone is a natural addition to the growing catalog of Chimani Guides. Because almost all the data is in the app, this is a hefty download of 357 megs. I keep the apps in iTunes, then download the ones I need for a particular trip.