Little known fact about me -- I am an amateur radio operator and have been dabbling in radio technology for about a decade now. I picked up the hobby when I lived in a rural part of Vermont with no cellular service. The only way to communicate over any distance was via ham radio using the local repeater network.
Repeaters are the lifeline of the ham radio operator. They are the towers that relay messages between ham radios, accepting the weak incoming signal and then transmitting it at a higher power, so it can cover a greater distance with minimal loss in quality. Depending on the location and power of the repeater transmitter, you can talk to someone a hundred miles away. And if that repeater is connected to a network, you can extend that distance even further.
One handy tool for the ham radio operator is an iPhone app, RepeaterBook from ZBM2 Software. The basic app takes the community repeater database of RepeaterBook.com and packages it into an iPhone-friendly format.
You can either browse through all the repeater entries for the US and Canada or enable location services and let RepeaterBook display a list of nearby repeaters. Each entry contains detailed information on the repeater so you can key in those details into your ham radio. The app stores its data on your phone, so you don't need an active Internet connection to browse through the repeater entries.
A handy filter allows you to show only those repeaters that meet your license class, radio type or preferred connections. For example, I have a basic handheld radio and set my filters to show only repeaters in the 2M and 70cm band. When you find a mistake with the repeater information, you can edit an entry and submit those details to the database administration.
The database is thorough for the area in which I live, but not perfect. It lists almost all the repeaters in my vicinity, missing only one less known repeater a few miles away. The best part about RepeaterBook is not what it does, but what I don't have to do now that I have it installed on my iPhone. Thanks to the RepeaterBook app, I no longer have to carry around my paperback repeater resource, which is bulky and comparatively inconvenient to thumb through.
RepeaterBook is available for free from the iOS App Store. It's compatible with the iPhone and requires iOS 5.1.