MileMarker app for iPhone and iPad

MileMarker uses drive-detection technology already built into the iOS platform to track your drives and give you monthly reports for your IRS deduction filings, and at $0.57 cents per mile, the value of tracking your miles adds-up fast. The best part about MileMarker is that, because drive-mileage tracking happens automatically, you will never again forget to log your trips.

Bonus:

Unlike other automatic mileage trackers, there is currently no monthly cost for ongoing tracking and reporting services.

Disclaimer:

Continued use of GPS running in the background can dramatically decrease battery life.