I'd say almost every system has a watchdog timer available, unfortunately drivers have to be written to take advantage of them.. usually it's a chipset feature (..southbridge), supported by the chipset driver.
However, there do exist some PCI cards..
berkwdt(4) was added a few releases ago, it's for Berkshire products.
A few of my systems have an
it(4) device, this is a temperature sensor.. but it also includes a watchdog timer that the kernel can use.
One problem is that if the interval is too short, the system might reboot.. even if the kernel may have recovered within a few seconds, also, the filesystem is left in an inconsistent state and you can lose data.