Boosting a WiFi signal can be an expensive proposition, requiring the purchase of either a second wireless router, or a wireless signal booster. Unless, of course, you build one yourself. From Instructables:
The parabolic Asian cooking (dumpling) strainer is the perfect candidate for this project. I was able to pick up 20 more access points in the city and connect to a network a few blocks away! This is BY-FAR the most simple of all Wifi extensions!
To build the booster, you need a USB extension cable, any USB WiFi adapter, and a tight-mesh parabolic cooking strainer. To reach WiFi signal heaven, stick the USB WiFi adaptor through the center of the parabolic strainer and connect it to the USB extension cable.
Wifi Signal Strainer [Instructables]