I purchased a temperature monitor for my computer after doing a few case modifications on it. It was a small little unlit screen that I could never make out the numbers to find out the temperature of my processor. Useless.
But this, this seems to be the way to go. AeroCool's PowerWatch has a neat display, easy to read front-panel readouts, brightly lit indicators (which only add to the allure of a case mod. in my opinion) with not only temperatures but fan speeds, too. Utilizing both Celsius and Fahrenheit, you can be in the know about your PC. Also has an alarm temperature warning in case you aren't watching its every move.
In addition to those bonus features it already impresses me with, the PowerWatch has two USB ports and four card-reader slots to make things more convenient. Using four different areas of your PC you can monitor all sorts of things while loading up things from your memory cards.
For around $75 you can add this to your list of gadgets you must have if you're into building your own systems. Comes in two color choices to make your design sleek.