I'm a iOS / Mac developer currently so I have a handful of iOS devices available to me ;)
Apps I use for motorcycling:
MotoMinder - Keeps track of service intervals and gas milage
GreatestRoad - Find good rides wherever you are
Campwhere - All state campgrounds stored on the device. Great for when you are looking for a place to stay but don't have any cellular connection.
NorthFace Trailhead - Kinda alright if you are looking for a place to hike or mountain bike.
iOS 4.2 broke MotoMinder for me so I hope they fix it soon.