Wireless communication between more than two Microchip PIC microcontrollers
The Nordic Semi nRF24L01+ is ideal for that sort of thing, low-cost modules are available on Ebay:

http://cgi.ebay.co.uk/Arduino-NRF24L01-Wireless-Transceiver-Module-2pcs-/280640828189?pt=LH_DefaultDomain_0&amp;hash=item41577f331d

The nRF24L01+ is often used in wireless sensor networks.

An MCU is required. I have a suitable design and test software here. It uses the much more expensive Sparkfun module, I've designed a board for the cheaper modules but haven't had one made for testing.