GPS app with a twist: iNavX + X-Traverse
iNavX™ - Marine Navigation App for iPhone
iNavX is interesting on its own, it lets you get government sourced (NOAA) charts of lakes, rivers, etc. in the USA and then use the iPhone GPS to navigate... and you exchange data with computer apps act as a repeater for boat safe navigation hardware (AIS), lots of other features.
What gets even more interesting is when you link this in with X-Traverse.com from Fugawi. The X-Traverse site lets you move GPS waypoint data (or other types of data) from your desktop computer to the iPhone or vice-versa using the iPhone's internet connection -- look ma, no cables, no nasty wire based synchronization hassles.
X-Traverse is starting with iNavX but it looks like it is extensible, that other iPhone apps could also leverage X-Traverse.com for any app that needs storage and transfer of data.