We use these and the higher power, super long distance, cousins for the majority of our radio needs. XBee has been around forever and their products just work.
Simply hooking up the in/out to your UART pins on both sides and your projects are now talking! There are advanced modes as well for power users that allow for sophisticated networking.
Make sure you get more than one if you don't have any yet.
This is the XBee XB24CZ7WIT-004 module from Digi. This is the updated version of the Series 2 (XB24-Z7WIT-004) module. It has an updated firmware which can be configured as router/endpoint/etc. without having to load a different firmware every time.
Series 2 modules allow you to create complex mesh networks based on the XBee ZB ZigBee mesh firmware.
These modules allow a very reliable and simple communication between microcontrollers, computers, systems, really anything with a serial port! Point to point and multi-point networks are supported. These are essentially the same hardware as the older Series 2, but have updated firmware. They fully compatible with the Series 2 modules.