Sparkfun RF Link Transmitter - 434MHz

 Since these are only transmitters, they will only work communicating data one-way, you would need two pairs (of different frequencies) to act as a transmitter/receiver pair.

Note: These modules are indiscriminate and will receive a fair amount of noise.  Both the transmitter and receiver work at common frequencies and don't have IDs. Therefore, a method of filtering this noise and pairing transmitter and receiver will be necessary. The example code in the Resources tab shows such an example for basic operation. Please refer to the example code and links below for ways to accomplish a robust wireless data link.

Features:

• 434 MHz
• 500ft range (given perfect conditions)
• 4800bps data rate
• 5V supply voltage

Useful Links:
Good AVR Tutorial