Android: If you ever lose your phone and don't have a contingency plan in place, you can remotely install Plan B on your phone and locate it instantly.
We've featured a few ways to find your lost or stolen phone, but they all require that you set up the system beforehand. Plan B, on the other hand, lets you remotely install it using the Android Market Web and immediately sends you the phone's coordinates after it does. You don't need to configure it, create an account, or do anything else — its location will just show up in your Gmail. It'll track your phone for 10 minutes, emailing you updates if it moves, after which you can just text the word "Locate" to your phone to track it again. For some phones, it can even turn on GPS if it's turned off (though it'll need an internet connection if you're installing the app remotely).
It doesn't have any of the data wiping or other mobile security that something like Prey would, but if you never installed a security app to begin with, it's a good second plan of attack in finding your phone. Plan B is a free download for Android 2.0 and up.