Quick Spoke app for iPhone and iPad
Spoke Length Calculator.
This is the one you want. Huge list of hubs and rims with predefined measurements. Multiple ways to measure hubs and rims. Save all the wheels you build so you have a reference library of spoke lengths for each wheel. Beautiful graphics assist in measurement specifics.
Multiple measurement and entry method. For example hub flange spacing can be entered as distance from locknut or from center. Rims can be specified by ERD or circumference and spoke bed depth. Specialty fields like offset spoke holes, and custom hub spoke hole sizes.