Raphael Cerqueira wanted a way to transmit data reliably and cheaply, so he developed a simple protocol to use with cheapo wireless transceivers. The basic idea is to add some identifying information and a checksum to each sent message, and require that the device receiving the message send an acknowledgment that it has been correctly received. This ensures that the message is actually received, and that it came through uncorrupted. He has source code implementing the protocol both as an Arduino library and a Java program for PC, and a more in-depth look at the protocol that explains how it works.
This seems like a nice and simple way to send quick messages between an Arduino and a PC, especially compared to the 802.15.4 protocol used in devices such as the xBee. Of course, the more advanced protocol is capable of many more things than this one, however sometimes you just need to get a message from one place to another, and his protocol seems like a great choice for those occasions.