The Navionics ap turns your phone into a contour mapping GPS for $10. That's about the best deal going right now. Just be careful, smart phones are not waterproof or rugged like your average handheld GPS.
Weather aps like Weather Channel, Weather Bug, and Weather Underground are great for keeping an eye on the conditions.
The camera is great for text-bragging fish to your friends who choose to stay home on the couch.