This is an easy method to conduct quick pin/port tests with a microcontroller. It’s helpful for debugging purposes and can be used many times without needing to wire up a breadboard. I have a few of these scattered around the workbench, allowing me to perform quick checks.
Simply solder a limiting resistor to the leg of an LED. Any ohm between 150 and 330 works well. It doesn’t matter which leg it is soldered to, but it is helpful to keep the anode (+) leg slightly longer than the cathode (-) in order to easily tell them apart (remember, many LEDs also have a flat molded into the plastic lens).
Here is a photo of the test LED plugged into a SparkFun Tiny AVR Programmer