Xbee
From DD-WRT Wiki
Xbee Serial Port Radio Link
Using AdaFruit Xbee carrier boards, you can add a Xbee radio link to the router. The radios I am using are rated for a maximum distance of 300 feet and cost around $20, however, you can upgrade the radios and there are some that go 1 mile, but cost twice as much. SparkFun electronics has a greater inventory of radios.
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)
XCTU software to configure the radios(http://www.digi.com/support/productdetl.jsp?pid=3352&osvid=57&tp=5&s=316)
Make the Xbee Carrier Boards Again, visit the AdaFruit site and user her instructions for making the boards(http://www.ladyada.net/make/xbee/index.html).
Configuring the Radios The AdaFruit site has a link to configuring the radios which is excellent (http://www.humboldt.edu/~cm19/XBee%20setup.pdf). Read through the document and setup one radio as the coordinator and the other as the end device.