In bread boarding a micro controller circuit, one of the most popular components is an LED. Light up an LED to indicate power connected, turn it on or off based on a mode change, or flash an LED to indicate the CPU is working. Because LEDs are typically rated for less current than a CPU or power supply can provide, they require a current limiting resistor to keep them from burning out.
One of my tasks in learning about new processors, is to see if I can configure every line as an output port, and if I can run the CPU at its maximum speed. I set up an integer counter in my code, and bit shift the value into all the output lines I care about. Using this LED bar, I can insert it into the breadboard next to the CPU pins, flip the switches off to lines I don't want to bother or mess with (like crystals, voltage regulator configuration, USB lines, etc.) and if everything works out right, watch the lights blink on and off in binary code.
The LED bar has nine LEDs in a row encased in green plastic, with 18 pins (upper pins are the cathode and the lower pins are the anode). Nine current limiting resistors connect the cathodes to ground through eight switches. The rightmost LED is used to indicate power and bypasses the switches; the jumper in the upper right is the ground (top) and power (bottom) connections. These components are placed on a mini circuit board with pins so it can sit upright on a breadboard.