Today as an autism mom, I'm beyond thankful that housing has a free alarm system, so our door chimes really loud any time my kids open it. We got the chime activated a couple months ago after Garrett figured out our safety latch at the top of the door and decided to go swimming by himself one day and to walk to McDonalds the other day. At least now, there's a really loud noise to let me know when he's feeling adventurous.
For those of you that don't have an alarm system, you can find several different types of door chimes/window chimes at your local hardware store or on Amazon.com. The best deal that I found was on Amazon: