Antenna on the Cheap (er, Chip) article shows how to build a simple “classic” WLAN antenna from Pringles potato chip can. I have built antenna based on this plan (some improvising) and I can say that it worked. Besides WLAN I have used it for wireless video at 2.4 GHz.
Cantenna article tells that in reality Pringles tube is too narrow to be practical. However, a cantenna can be made from various cans or tubes of an appropriate diameter.
802.11b Homebrew WiFi Antenna Shootout has some interesting reading on various DIY WLAN antenna options and how well they perform.
How to boost your WiFi signal with a beer can shows with several pictures how a beer or soda can can be converted to a WLAN signal booster antenna.