Now available from PMD Way is the useful MAX813 Watchdog Circuit IC in packs of ten. ICs are through-hole, DIP8 packaging.

The MAX813 can be used to build a "watchdog" circuit, that offers:

1. A reset output during power-up, power-down, and brownout conditions.
2. An independent watchdog output that goes low if the watchdog input has not been toggled within 1.6 seconds.
3. A 1.25V threshold detector for power-fail warning, low-battery detection, or for monitoring a power supply other than +5V.
4. An active-low manual-reset input.

In other words, your microcontroller can automatically be reset if there is a power supply issue, or software/code issues that cause the microcontroller to lock up. 

For more information, please download the MAX813 datasheet (pdf).