Real Temp / Core Temp

There are a whole load of programs that will monitor the temperature sensors in your computer for you (and some that provide a whole load of tweaking, from over-clocking to greater control over fans).

The only program like this I currently use is Real Temp, it allows me to display the temperature of my cores in the system as well as set alerts for when they get to warm in the summer. It isn't incredibly light on system resources and seems to give accurate readings. Unfortunately it only claims to support Intel chips (which isn't a problem for me).

For AMD chips I recommend Core Temp, it supports most chips and is equally light weight.