Folks,
I've got an Arduino project that needs to be very reliable. I have added an external watchdog board that I got from SwitchDoc Labs (Dual WatchDog Timer - SwitchDoc Labs Blog).
It works like a champ. The Arduino Mega has about 5000 lines of code and would hang periodically in marginal voltage conditions (it's solar powered) and never recover. Hooked up this board, added the reset code to the program and the board would reset the Mega when it was lost.
Now it really is much more reliable. Here's a fritzing diagram from the website on how to hook it up to an Arduino.
Faraday