You've misplaced your cellphone in your house and you don't have a land line or friend handy to call your number and make it ring. If you've got internet access, WheresMyCellPhone can fill in for your friend.
The site is no-frills, for sure. You plug in your phone number, hit the "Make it Ring!" button, and WheresMyCellPhone dispatches a call to your wayward phone. You can only use the service once every few minutes—presumably to prevent less honorable uses—so make sure you're ready to dash towards the ringing when the call comes in.
Have a trick or two for finding a misplaced cellphone or ensuring it doesn't get misplaced in the first place? Let's hear about them in the comments.