Probably you already have few cheap RF transmitter and receiver modules around. If not you can grab them at Sparkfun for really cheap. So one thing is great they are cheap, but these are only one way 315MHz modules, so signal can be transmitted one way – ideal solution for some kind monitoring like weather stations and so on. These modules are capable to transmit data at up to 2400bps rate to a max of 500 feet distance.
So here it is a simple set up of two Arduino boards – one with receiver and another with transmitter module. As these modules don’t include any transmitting protocol, it has to be programmed within microcontroller. Transmitting protocol basically consists of two byte starter packet with helps to prevent random noise, then follows address byte that allows addressing multiple transmitters, 2 bytes of data, and last portion is a check sum – to make sure packet isn’t broken. So this is a quick example for your next project if you decide to use these modules.