Using an old Wireless Router as an Access Point or Range Extender.

Surprisingly easy to do, if you know how.

http://www.makeuseof.com/tag/how-to-turn-an-old-router-into-a-wireless-bridge/

http://www.dslreports.com/faq/11233

See also:
http://www.dd-wrt.com/wiki/index.php/Wireless_Access_Point