If you run out the door every morning without remembering to lock it (or remembering if you did), this little DIY project can help remind you to lock the door by simply buzzing when you forget.
DIYer pppd put this together using nothing but an ATtiny13, two reed switches and a buzzer. Even if you're not super experienced with electronics boards, it should be a simple device to put together -- and the peace of mind it can give you as you rush off to the bus in the morning is priceless. Hit the link for more details on how to build it. Now if only someone could build the same thing that tells me when I've left the oven on, too.