Find My Phone is a free app that can help you locate your missing iPhone 4, iPad, or fourth-generation iPod Touch, and it can even remotely lock or wipe all data from the device. This free, app-assisted service (which requires iOS 4.2, as well as setting up a MobileMe Free account) was previously available only to paid MobileMe subscribers.
Not surprisingly, you will likely never use this app from your device itself, but rather from your Mac (you just go to me.com/find in your browser) or from another device (one that you own, or a friend's--any device that has the app installed will work). Whether you're using your browser or another device, Find My Phone has a similar interface: once you enter your MobileMe username and password (or AppleID), you see your missing device's approximate location, determined via GPS (the accuracy of which, of course, can vary greatly, but it's typically good enough to let you know whether your iPhone is in your house, at a bar, or on a fast-moving bus). You can then display a message or play a sound on the device (a piercing sonar beep that's easily heard even through couch cushions, even if your mute switch is on), lock it up (you can set a new passcode), or wipe your phone completely (which resets your phone to factory settings, erasing all data--including the Find My Phone app).
The most cumbersome part of Find My Phone's interface is the app's setup (which can include signing up for a MobileMe Free account, and agreeing with 14 pages of Terms of Service), but it's very easy--and even kind of fun--to use once it's up and running. And this being an official Apple app, Find My Phone has good documentation. If you own one of the newer devices that's eligible for Find My Phone, you might be very thankful one day that you installed it.