iHandy Carpenter offers a virtual ruler that can be swiped across a surface for measurement across (though requires the aid of a finger, so we think this isn't as useful as it could be), a bubble level bar indicator, a plumb bob to measure the verticality of a wall, steel protractor to measure angles from 0-180 (we're having bad flashbacks to jr. high geometry classes) and a surface level. All measurements can be fine calibrated for exact readings.
The iHandy Level app sans other measurement features is offered for free while the full iHandy Carpenter app is offered for $1.99 from the App Store.