As you probably know, there are some pretty cheap RF transmitters and receivers around. For something like $10 you can get a transmitter/receiver pair and get one-way communication link going.
With most RF projects like that you will need to implement some level of error correction. In most cases a simple checksum is enough, but there are situations where it is necessary to implement a full reliable wireless communication protocol guaranteeing that the message is transmitted properly.
This Arduino project utilises a couple of the aforementioned cheap RF transmitters and receivers, establishing a two-way link on different frequencies and describes the synchronization protocol.
[ Project ]