Drive for long enough, and eventually you'll experience it: that helpless feeling that comes when you discover you've locked your keys in your car. Before it happens again, install a system that unlocks your car doors with your cellphone. The setup involves using the Bluetooth module from a cheap wireless printer, which receives a command from your phone and sends it to a switch connected to your car's spare electronic key fob. Instead of telling the printer to print a photo like it normally would, the phone signals it to trigger the unlocking mechanism. Stash the system under a seat (plugged into a lighter socket for power), and a few keystrokes will pop open the doors from up to 10 feet away.