Pintail is an application for Android phones. It was an experiment in "useful minimalism". It's now slightly less minimalist, but rather more useful.
Click on the images below for a full-sized view.
Pintail is available from the Android Market
Just launch the application and enter a PIN number into the box, then when you need to find your phone, borrow someone else's and send yours an SMS which includes the text:
eg. locate 256
The phone will do its best to pinpoint its position before sending back an SMS message containing its location and a handy link to Google Maps.
If you want to, you can share the PIN number with others. They can use it to find out where you are when they message you, but please share it responsibly!.
You can even leave the PIN blank but this is potentially unsafe since anyone could request your location, but you'll always know they've requested it because you will be notified of the incoming SMS.