Grit is OpenLongTrails.org's free mobile app for hikers on long distance nature trails. It contains lists of waypoints for various trails -- water sources, campsites, resupply towns, etc. -- and uses your current location to show where you are along your selected trail. Each waypoint has a details screen with more information and a comments section, where you can read recent updates from other hikers and add your own comments.
List of currently available trails:
- Pacific Crest Trail