Temperature Monitor is an application to read out all accessible temperature sensors in Macintosh computers. The program can visualize the history graphs of the readings, display measured values in a large variety of fashions, or announce readings by speech output. It can also store and export values. Temperature Monitor is the small gratis version of the application Hardware Monitor. Hardware Monitor can additionally display "non-temperature" sensors and artificial operating system probes, or control external LCD panels. Temperature Monitor is part of a software suite of 9 different monitoring applications which can be used for numerous purposes. Temperature Monitor is electronically distributed software. You can download the product free of charge.
… More
Edit
Great tool. I can get a clear idea of when my computer is going to start acting up (fan whirr) based on the temperature. I also know when things are getting too hot now.