A clever 15 year old developer named Nick Lee got one over on the App Store team by releasing an innocent looking app called Handy Light. For 99 cents, you could grab his Handy Light app, which allows you to make your iPhone screen go blank with the color of your choice to use as a "flashlight." Unknown to the App Store folks though, Nick also included code that made Handy Light run as a full tethering app, which allowed the iPhone to be used as a 3G wireless modem for free; something AT&T makes you pay an extra $20 a month to do.
Apple got wind of the alternate use for the program and took it down from the App Store, so this is no longer an option for those that did not grab the app before the App Store got wind of what was going on. It does beg an interesting question though; what secret unknown functions do the apps you downloaded have?