MotionX ($1.99, iOS only)
MotionX is designed as a multi-sport outdoor GPS app that many mountain bikers use in lieu of a dedicated GPS device. Beyond offering the standard readouts on distance, time, speed, and elevation (plus charts plotting metrics over time), MotionX offers the ability to download basemaps before you head out of cellphone range. This way, you can utilize maps for navigation no matter how far out you travel!
Use this if: You want a GPS app that can help you navigate unfamiliar terrain. This app gets pretty close to a full-fledged, top-of-the-line GPS unit, minus advanced training metrics or the ability to pair a heart rate monitor or power meter.