Real geeks have a fancontroller/case thermometer installed in a 5.25" bracket in their case. These will set you back at least $30/€25. This DIY project will instruct you how to make a case thermometer for less than $3!

Material needed

CD/DVD drive housing (free I assume)
LCD thermometer (DealExtreme, with backlight $6)

Tools needed

Flathead or Phillips screwdriver
Saw
Sanding paper
Ruler

Step 1: Cut a Hole to Fit Your Thermometer

I assume you know how to 'hollow out' the CD drive, it usually comes down to unscrewing everything you can find. There's a good instructable on it here. Make sure you keep the screws for fastening the drive to your case, and the ones keeping the housing together.

Now take of the front of what used to be a CD drive. Measure the outer dimensions of your thermometer, and mark it on the CD drive. Then saw it out to those dimensions with your. It's easier if you just cut off the top bezel partially, it doesn't contribute much to the structural integrity of the front anyway. As usual, it's better to saw it too tight and sand it later, than to cut of too much. If you do it precise enough, it will fit tightly without the need for glue.

Step 2: Place the Sensor Inside Your Case

Now lead the sensor wire through the back of your CD drive housing and put it at the desired place in your case. Make sure it won't get stuck between fans and cause havoc. Use of tie-wraps or equivalent is recommended.

Step 3: Test It and Some Improvements

Now you have to do the tedious job of testing it. Start your computer, and find something that will warm up your PC, like a heavy game.

In my case, having a 150W AMD GPU made the temperatures rise quickly, but it was nothing to panic about.

Improvements

The result for now isn't a looker and it doesn't offer a lot of functionality either.

I don't need a fan controller myself, but if you want one, check out this instructable for example. You can also add a second thermometer and place that sensor somewhere else in your case.

Personally I will try to make it powered from my PSU so that I wont have to worry about empty batteries, add some LEDs, and close off the front entirely, painting the housing black.