Little known fact about me -- I am a ham 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 without degradation. Depending on the location and power of the repeater transmitter, you talk to someone a hundred miles away. And if that repeater is connected to a network, you can extend that distance significantly 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 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 ham 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.