Writing code and toggling pin states to high or low is all good and well in the land of fairy tales where everything works perfectly the first time. In real life when you write some code that doesn’t behave as expected, it is extremely useful to have some debugging tools at hand. Tools like an oscilloscope, logic analyser, and multi-meter all have their uses but are overkill if all you are trying to do is find out if a pin is at 0V or 5V.
The Logic Probe Kit made by Jdn comes with three LEDs that each represent a certain portion of the supply voltage. For example for a 5V supply, green turns on for 2.4 – 5V, yellow turns on for 1.7 – 2.4V and red for 0 – 1.7V. So at a glance you can tell if a pin is high, low, or somewhere in the middle (possibly indicating something is drawing too much current from it or it is floating).
Not only can you easily look at pin states, the pen also has some extra CMOS circuitry built-in to pick up 100ns glitches. This is a useful feature that might indicate something like a microcontroller that is continually resetting.