Last week I downloaded the AccuTerra App for the iPhone, and wow... it ROCKS! Hands down, I think it's the best app for hikers or outdoor adventurers. They have a huge catalog of maps you can download directly from the app. Some maps are free (usually parks and rec areas) but others you have to buy, however they are fairly cheap ($2-3 for a large area). The best part is that this app works even when your phone has no service. We were camping/hiking in the Pike National Forest this past weekend and had no cell phone service but it was still able to track us. I love how you can save your "adventure", and even share it by email or on facebook. It keeps your bread crumbs, allows you to take notes, and mark places of interest. You can also take photos while recording so they are geo-tagged and shown on your map where they were taken. LOVE it!
The version of AccuTerra I downloaded was free, but now they also have a version called Map Candy where for $2.99 (limited time) you get the app and unlimited map downloads. Since I've already spent $1.99 for one map of southern Colorado... I think it's probably worth it. In case you're wondering, no AccuTerra is not paying me to advertise for them, I just really love their app! :)