How do I find my CPU's temperature?
While iStat Menus @Mark mentioned are no longer freeware, the iStat nano and iStat Pro Dashboard widgets are free.

I personally use iStat nano for my system monitoring needs. Here it is showing the temps it can read on my iMac:



If you hover over the widget, you can change the view. I usually have it running in "Overview" mode, which shows CPU, RAM, HDD*, fan*, temperature*, network* and uptime info. 

*) user-configurable, e.g. which temp sensor's readings are displayed