Delivery Status is a gorgeous OSX-only Dashboard widget and iOS app that allows you to easily track packages from a huge list of carriers (international and domestic) and companies like Amazon and Google Checkout. On the Mac, you can be notified of status changes via Growl, and on iOS it uses push notifications. If you use both, as I do, packages are seamlessly synced.
Data entry is flexible, with perhaps the easiest being a bookmarklet that automatically parses the order status page of most shippers (and many retailers). I frequently shop online, and I send a lot of packages, so I use it to track those that are incoming as well as outgoing. As a city dweller, knowing that a package has been delivered is essential, so that I can pick it up from the lobby of my building before it “walks away.”