Newer motherboards have at least two thermal sensors on board. One directily under the CPU, and one somewhere else on the motherboard. Fan speeds and various system voltages are also usualy available.
Dump of data from my system:
Code:
VCore 1: +1.64 V
VCore 2: +2.75 V
+3.3V: +3.40 V
+5V: +5.07 V
+12V: +12.16 V
-12V: -12.03 V
-5V: -5.32 V
V5SB: +5.53 V
VBat: +3.63 V
cpu fan: 3924 RPM
vid fan: 5720 RPM
mb temp: +25°C
cpu temp: +36.0°C
If you're using windows, check out
motherboard monitor to read these sensors.
As to actualy making one of these, the cheap and easy way to do it is to use the joystick port, because it can be hooked directly to a thermisistor.