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.
I've tried many of the others but TMS works great. Any eBay or email notifications that you receive that include a tracking number can be simply emailed to TMS and they are automatically added to the iPhone app.
There is also a companion website should you wish to check via your computer.
Well while we are at it. I may as well let you know of my app called Posted and Posted Lite which has been on the App Store for about 6 months. It supports a lot more providers than some of the others mentioned here (around 40 from around the world including many from Australia). As well as push notifications and barcode scanning.
There is a free version with all features (except push) and a paid version for a couple of dollars.
I am also very actively developing it and have some great feedback due to responding to users feedback so quickly.