I travel around town when I photograph homes for TourFactory so mileage tracking is important.
I just recently downloaded MileIQ for iPhone. It will automatically track mileage for all travel from one point to the next. You just swipe left or right to choose Personal category or Business category. The only thing is you're limited to how many drives you can track per month with the FREE version.
You can also login to your account online to see your miles, edit drive details, and create expense reports.
It totals your mileage and dollar amount for your business drives. So far I'm loving it!