Xbee
From DD-WRT Wiki
Xbee Serial Port Radio Link
Using LadyAda Xbee carrier boards, you can add an Xbee radio to the router.
What you will need: 2 Xbee radios(http://www.adafruit.com/index.php?main_page=product_info&cPath=29&products_id=128)
2 Xbee carrier boards(http://www.adafruit.com/index.php?main_page=product_info&cPath=29&products_id=126)
1 USB FTDI cable(http://www.adafruit.com/index.php?main_page=product_info&cPath=29&products_id=70)
Female jumper wires(http://www.sparkfun.com/commerce/product_info.php?products_id=9389)