Best choice for low-power, short-range wireless communication
Take a look the XBee line of tranceivers. There are various types depending on the transmission range and desired power rating. 

These devices also interface with Arduino projects nearly trivially. The Serial library makes use of the AVR's UART hardware which is directly compatible with the Rx pin of the XBee. Also note that no level shifters are needed because the XBee Rx/Tx pin work with 5V! 

DO note, however, that the XBee must be powered with 3.3V, so a simple 3.3V LDO voltage regulator can be used.