First and foremost, AllTrails. I use it on my IPhone for finding trails, navigating trails, reading reviews on trails; my favorite. Never hike without it running.
UPDATE 4/5/2014: I’ve recently decided to stop using AllTrails for my navigation. I’m tired of it crashing on me when I’m hiking causing us to get lost (yes we have paper maps and a compass) or causing my overall track be inaccurate. I guess I’ve just outgrown the app. I still will try and upload my tracks that I create in Gaia GPS to it and my reviews/photos so others can find information about trails but for actual navigation, I’m not using it anymore.
It’s pros are that it’s more of a community-based app. Great for finding trails, reading reviews by others, looking at pictures but the cons are that it is too hard to get to the features you most need when your hiking. I had a paid subscription to AllTrails that I definitely will not renew. It was not worth the $50 a year.