Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
If you mean an app to check the internal temperature of the processor, Core Duo Temp 2 is a small app that does it.

TemperatureMonitor:

http://www.bresink.com/osx/TemperatureMonitor.html

Will tell you the same (either in an app, your menubar, or in Dashboard), plus the temperature reported by the hard drive's internal sensor (assuming it has one).

If you're wondering about reading the internal sensor that Apple uses to control the fans, nothing does it yet.
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.