Living with weak Wi-Fi is no way to live. If you're not in a place where you can just add a bridge or repeater, a quick trip to the kitchen may be in order. You'll need a baby bottle, a mesh strainer and a USB Wi-Fi dongle. Here's how the whole thing works.
Stathack explains the whole project over at his blog (linked below), but essentially he managed to turn the mesh strainer and the baby bottle into a directional parabolic antenna so he could mooch Wi-Fi from the coffee shop down the street. Of course, if you're planning to do the same thing, you may want to consider mooching off of your router that your computer is too weak to connect to, or another network you're actually authorised to use (just for safety's sake).
In addition to the kitchen gear, he also picked up a USB Wi-FI dongle, a USB cable, glue, electrical tape and wire cutters. He explained that all the parts set him back about $US50, and that was because he didn't own any of them in advance. Building the whole thing was easy enough too — he attached the baby bottle to the strainer (after doing a little maths to find out the best position), attached the USB cable to the dongle and mounted it in the baby bottle, taped the whole thing to a mop and put it on his porch, and then plugged the USB cable into his computer. Sure enough, he was able to pick up Wi-Fi signals up to a kilometre away.
For more detail on the project and how the whole thing was made, hit the link below.