AirPort Utility is a free tool from Apple that enables you to use your iPhone or iPad to manage your wireless network including your AirPort Express and Time Capsule. Used at its most basic the app can be used to view a network map so you can see at a glance which devices are currently connected to your network, but it is also capable of so much more.
Should you find that your base station is becoming unstable, you can use your iPhone or iPad to perform a quick restart without the need to move to your Mac or PC. As iOS devices, the iPad in particular, have turned into laptop replacements for use around the home, especially when slumped on the sofa, this is a useful tool for keeping on top of your home network.
As well as being able to view the devices that are connected to your network, you can also use the app to change security settings and control network options such as wireless channels. It is also possible to upgrade firmware, control port mapping and manage DCHP reservations.
Apple had originally intended for this feature to be an integral part of iOS 5, but the decision was taken to release it as a separate app. iOS 5 is still a requirement for installing this universal pad and while it is something that may prove useful from time to time, this is unlikely to be an app you use on an daily basis.
Handy for occasional use, but tinkering with network settings is not something many people do with any regularity from iOS.