Great little application that takes virtually no resources
What it does:
It gives you detailed information on your CPU load, as well as the temperature of the hottest point in your CPU.
Also, when minimized, it shows up in the system tray as a red number, and that is the temperature. I love this feature, it's good for quickly seeing how hot your CPU is running.
And there's a Desktop Widget addon for it which is also for easy viewing, but gives a bit more detailed info
This application only shows your temperature for the CPU, not any other components like GPU or etc, but it's probably lighter than the ones that do :P
I love this, its great for me because I want to see how hot my CPU gets when doing certain things. So far the hottest it's gotten is 30° C, and that was while watching an HD video, downloading a torrent, and installing a game http://www.alcpu.com/CoreTemp/