NavLink US is a full-featured chartplotter app that is designed for both the iPad and iPhone. Visually, it has a nice, clean look and intuitive controls, and can be used for both trip planning and navigation. The app comes with all the NOAA vector charts and presents them in high-res. Charts can be shown as north up or course up, and your current position, track, course, speed, ETA, VMG, and bearing and distance to next waypoint are all shown on the screen. A tap on an object such as a buoy or marker will give you its details. NavLink can also link to AIS systems and present that data as well. A version for the UK is also available.
NavLink US is available for $19.99 from the iTunes store.