A week ago my old iPhone 3GS stopped getting service. No matter what I tried I could not get a signal. Reluctantly I gave up on trying to make it useful as a phone but since everything else still worked perfectly I thought there still must be a good use for it. After a little research I now use it as a cheap security camera that sits in one of my garage windows and monitors anyone approaching my house from the street. The basic app "Manything" is free on iTunes or you can purchase a more extensive version. I can be at work, login into the app's website and see in real time what is currently happening or go back and look at recorded "events" (when there's motion in the monitored area). Definitely not a replacement for a quality security camera but if you want a cheap extra set of eyes at home when your away this is pretty good.
Anybody know of any other uses for old cell phones? I happen to have a couple more collecting dust.