Building my own HDTV antenna out of aluminum foil reminded me of a few years back when I needed to improve my WiFi signal, and I built foil reflectors to do that as cheaply as possible. There are a bunch of designs out there, but I like the EZ-12 parabolic reflector from freeantennas.com since it's quick, cheap and easy to make. It's an aluminum, paper and tape solution to weak WiFi signal strength.
I needed to boost the wifi signal going out to a Linksys WRT54GL running modded Linux firmware and running as a WET bridge so it could punch through a glass window and get a network connection to a bunch of wired devices. I made two of these parabolic reflectors out of aluminum foil and paper, stuck them on the antennas and they worked spectacularly.
Basically I was sharing my wifi signal with my neighbor and his computers were in the basement, which was a problem. Since the wifi couldn't punch through the basement concrete or the ground we placed a WRT54G running the old HyperWRT Thibor firmware (I'd recommend Tomato for people doing this now) in WET bridge mode by the basement window to pick up the signal from my house and get a connection to the wired devices plugged into the WRT54G. The signal was too weak to get through the heavy glass window, but with the foil reflectors placed on the antennas everything worked great. A little bit of aluminum foil made all the difference between having a working setup and having no wifi at all.
The signal strength with the foil reflector added on was strong, and the wifi connection speed came close to the speed of the broadband connection I was using. Pretty good for a cheap router and some metal foil!
The EZ-12 Parabolic reflector design is nice and simple. It's like a dish that forces the antenna to send and receive a signal in only one direction, making the wifi signal stronger as a result. It's a simple solution to an annoying problem, and it's so cheap and easy that adding aluminum foil is now the first thing I try when faced with a weak wifi signal at a location. It looks a bit rough, but it's clever and it works.