I have a Droid Charge, but I live overseas and it's not an international phone so I can only use it for wifi. It is constantly searching for a signal that doesn't exist and it's eating my battery up! Is there a way to turn off the cell signal while keeping the wifi on?
I don't know for the Droid Charge in special, but on many phones you can turn off the radio separatly. And even if that doesn't work, in your case it should be fine to simply enter airplane mode, and then activate WiFi again (plus Bluetooth, when needed) -- which eventually has the very same effect.
I remember having used Dazzle Configurable Switcher for this on my old HTC Wildfire (to turn off radio separatly), and it worked like a charm. On my Motorola Milestone 2, however, I was not given that choice -- but could use the work-around described.
Set airplane mode.
Under phone setting, enable airplane mode. When this happens your phone's WiFi will turn off, but you can activate WiFi again.
Airplane mode ONLY disables your phones radio. It usually turns off other communication features such as WiFi and Bluetooth but these can be turned on again.
There is also a detailed article on using an Android phone as a WiFi only device (no cellar connection).