You can use the BCM2708's hardware watchdog.

To use it begin by loading the module:

sudo modprobe bcm2708_wdog

Then edit the /etc/modules file:

sudo nano /etc/modules

and add the following line:

bcm2708_wdog

Next you will need to setup the watchdog daemon.

Install and confiigure it to start on bootup:

sudo apt-get install watchdog chkconfig chkconfig watchdog on sudo /etc/init.d/watchdog start

Next configure watchdog:

sudo nano /etc/watchdog.conf

Uncomment the line #watchdog-device = /dev/watchdog so it reads:

watchdog-device = /dev/watchdog

The watchdog daemon will send /dev/watchdog a heartbeat every 10 seconds. If /dev/watchdog does not receive this signal it will restart your Raspberry Pi.

This can be useful if you are accessing your Pi remotely, and it dies or locks up. However, this is not the preferred method of restarting the system, but can be used to restart a locked system, where the only other option is to remove power from the device. Be warned that this can result in filesystem damage that could prevent the Pi from booting and operating correctly.

More info including a method to test this setup can be found in Gadgetoid's blog post Who watches the watcher?. Binerry's tumblr post Raspberry Pi Watchdog Timer should also be a must read.