Dropbox is pretty awesome, as is the unit converter.
Not useful, but a great time waster: Asphalt 7!
https://itunes.apple.com/us/app/asphalt-7-heat/id462694916?mt=8
Level (it is actually a level app. Not super accurate, but better than eyeing it)
Shazam (identifies songs playing wherever you are)
Skitch (allows you to annotate just about anything on the fly)
Autoblog
NBC (stream TV and news)
Uber (allows you to call a Lincoln Towne car to pick you up in most major cities, costs just a few bucks more than a cab in most cases and is great for getting to and from airports in busy cities)
LinkedIn (which acts as a nice contact book for me on the go)
Yelp
Open Table
iWant
moviefone
My definition of useful may be skewed…