An update to the popular DD-WRT firmware for wireless routers can turn the device into a signal repeater that extends your current network's range. The Hackszine blog points out a tutorial for doing just that:
This How-To provides step-by-step instructions for creating a Universal Wireless Repeater appliance: a device that you can place anywhere and it will wirelessly repeat the strongest signal, onto another wireless network (with or without security). This functionality is also known as Wireless Client Bridge, or Range Expander. Unlike WDS, once you have this appliance setup, it will work with any open network.
Sounds like a great way to extend your network from the office to the basement. See also Adam's tutorial on how to turn a $60 router into a $600 router with the DD-WRT firmware installed.
Universal Wireless Repeater [WRT Wiki via Hackszine]