Core Temp Gadget
Full user review
"also need to download core temp software"
Free! and it works.
You have to keep the Core Temp software running on your computer in order for temperatures to be registered.
The software link is annoying to find because the default large font link on their webpage also installs freeware download manager that suggests other software for you to download. Instead, click on the More Downloads link and click on Standalone App. Then you don't get the additional spammy software installed with it.