BlueRetriever is the free web-based service that helps reunite you with your mobile device if you lose it, while allowing you to reward the finder for their good deed with a designated reward amount. The system relies upon creating a unique ID wallpaper, which allows anyone who finds your mobile to contact you anonymously.
Crackberry users can cut short any chances of handheld withdrawal using a myriad of options, including locating via an email message, putting the Blackberry into alarm mode to track it down audibly, vibration+flashing screen mode for those night time cell phone hunts, and GPS map tracking when you've really misplaced your cell. Free trial and then $6.95 for the full version.
Where's My Cellphone.com
Enter your number, set the time when you want for the call to be made (instant or between 30 seconds and 5 minutes). It's free and you can even use a QR code to automatically download via Android Market. The developer only asks you buy him a beer if successful.
Mobile Defense displays real-time location information of a lost or stolen phone on an interactive map. Activate an audible alert to further assist in locating the device even if the ringtone is set to silent or vibrate. It just went from closed beta to a wait list sign-up, so get your name on that list if you want to give it a go once they relaunch.
And do yourself a favor by designating a spot in your house/home office where you aim to always set your cell phone. Because prevention is the best measure against losing anything. A digital device "landing strip" and charging station makes sense for both recharging your device daily and also making sure you set it down in the same place every time.