I have seen some very innovative do-it-yourself (DIY) wifi antenna. One interesting directional unit used an aluminum cola can as its basis. Any case, do a google search and it will turn up many options.

Look here for the Asian cookware solution. (parabolic antenna.)

Look here for a set of papers on everything you wanted to know about 2.4 Ghz antenna theory and (seemingly) every kind of antenna that might be used, including DIY. It is a lot of really good information.

I have a three part post in my workbench creations blog on this topic, right here on CR4, Here are the links

http://cr4.globalspec.com/blogentry/2697/Build-Your-Own-Wireless-Networking-Antennas-for-Longer-Distance-802-11b-g

http://cr4.globalspec.com/blogentry/2763/Build-Your-Own-Wireless-Networking-Antennas-for-Longer-Distance-8-0211b-g-Part-2

http://cr4.globalspec.com/blogentry/2839/Build-Your-Own-Wireless-Networking-Antennas-for-Longer-Distance-8-0211b-g-Part-3

Alligator clip your computer to a honking big piece of metal.

I have seen the responses, and I just write to tell you "it works". One of my colleagues, wanting to work from his dorm room at our facility, made one of these and was able to connect effectively to the WIFI from about 150 meters away. The solutions are very directional, but he was able to "aim" the antenna well enough to solve his problem.

(he was also able to find several others from nearby buildings, but fortunately for them, they were all secure)

Good luck

Do you remember Quartz Crystal radios that worked without batteries and all you needed to do was clip it to a piece of metal? I spent many a night under the covers listening to very old reruns of Lights Out and the Whistler!