LM78 is a hardware monitoring chip made by National Semiconductor. It can monitor 5 positive voltages, 2 negative voltages, 3 fans speeds, a case intursion sensor, and a CPU temperature sensor. The lm78 measures voltages between 0 and 4.8 volts, so readings for the +5 and +12 rails are divided down, measured, the scaled by the software that monitors the lm78. Most computers built during or after 1998 have an lm78 or similar chip in them. An lm78 can interface with the system via the ISA bus or the SMBus (System Managment Bus)

The lm78 also supports alarms that can trigger an interrupt when a sensor goes out of a specified range. Most users, however, don't have a spare IRQ line to allocate to the lm78, and some motherboard makers don't even bother connecting the lm78's interrupt pin.

Software to monitor these chips is available for M$ Windows and Linux (via the lm_sensors kernel module). There are applets for just about every window manager that will display CPU temperatures and voltages on the screen.
full documentation of the LM78 chip is available on national semiconductor's web site at http://www.national.com