I like both TrackMyShipments and Consume (both in the app store).
Consume let you track a variety of things, your cell phone minutes & data as well as packages and other types of accounts, and it updates from the shipper's site each time you open the app. I've found it to be very accurate.
TrackMyShipments allows you to forward the email you got the tracking number in and it automatically adds that package for you. It also allows you to set up push notifications and such, but sometimes it's updates are right on target and sometimes there's a delay of up to a couple of hours.