I use Motion-X gps. App is cheap and downloads of terrain maps are free. I have downloaded maps for Peru and Nepal as well as the Sierra. Sometimes they are more accurate and detailed than Nat Geo maps or other competitors. I've been impressed with the accuracy.
I also have a variety of Tom Harrison maps and the JMT map pack on my iPhone, GPS-enabled apps. Nice to be able to idiot check my read of paper topo. Has helped me get better and more confident with paper topo.
