What Android/iOS app should I use for an odometer?

You can absolutely get by on stage rallies without a fancy dedicated rally computer. If you have a smart phone or tablet, there are a variety of odometer apps that will work. If you want to take it a step further, an external bluetooth GPS will increase your accuracy and reduce power consumption, but most likely, you won't notice the difference. Be sure to run with your device plugged in to power, and in airplane mode for distraction removal.

Android

Rally Tripmeter
ASE Rally Monitor 100 (also known as "ARM")
ASE Rally Monitor 110
GPS Tripmeter Non-rally-specific. Basic odo.
GPStrip Non-rally-specific. Simple interface
GPS Speedometer (has ads)
RallyComputer.ca $39.99 CDN
Richta Rally Odometer $5.00

iOS

Rally Tripmeter - Same as Android Rally Tripmeter above
ASE Rally Monitor $5.99
Track Addict
TrackPal $1.39