Milebug from iZATT is a premier iPhone mileage log application that helps those driving their own vehicles for business, charity, medical, or other reasons receive their reimbursement and/or tax deduction. MileBug for the iPhone or iPod Touch helps you track your miles and provides emailed reports for easy submission to employers, accountants, and the IRS. In the U.S., IRS business rates are up to 58.5 cents/mile.
If you want to give it a whirl first, pick up Milebug LITE which is free and is fully functional but is restriction to 10 trips.
This 3rd Party App is available at the Apple iTunes AppStore. Browse the full list of all AppStore apps filed under the AppStore category.