Platform: Apple iPhone, iPad, iPod touch
An app to find your lost iPhone sounds about as useful as a solar-powered torch. Perhaps that's why Apple is giving it away for free.
You start by signing up for a free MobileMe account using your Apple ID. This doesn't (yet) include any of the synching features in the paid service, and it takes just a second. After verifying your account, you turn on phone location in the MobileMe menu.
Now, every time you start up the Find My Phone app (it's password secured), it will hunt out any Apple devices you've registered, including iPads and the newest iPod touches.
It takes about 30 seconds to locate switched-on phones and tablets, and displays them in a map view. As you'd expect, the accuracy is usually good enough to see which building your phone is in, but it won't tell you whether it's in the laundry basket or under that pile of magazines on the kitchen table.
You now have three options. You can display a message and play a sound on the phone. The piercing ping should be loud enough to help you locate a lost (and otherwise muted) phone at home, or annoy someone at the pub enough to see what's making all that noise.
You can also remotely change the phone's lock code, and as a last resort, wipe it completely of personal data. You can also do all of these from the MobileMe website.
Ultimately, like a photovoltaic flashlight, this app could be pretty useful, especially for anyone with multiple Apple gadgets - or a parent looking for a sneaky way to keep a digital eye on their kid via an iPod touch.