Using an Arduino you can easily interpret data coming in from the serial line, and with that in mind enthusiast "kraegar" created a system to display the memory and CPU usage of his PC with easy-to-read LED bar graphs. The Arduino simply listens to the serial line for the values from the PC then converts the values suitable for driving the LEDs via 74HC595 shift registers. It's simple and it works.
That's a fun and interesting project, you can follow the instructions from here. And for more, we're on twitter and Google+, so follow us for news and product updates as well.
When working with 74HC595 shift registers and solderless breadboards (or more permanent solutions) consider using the Freetronics EXPAND: Expansion/Shift register module. It contains the 74HC595 shift register on a small board with standard 0.1" spacing holes that are perfect for soldering header pins into for breadboard use, and a small "power on" LED. By doing this you have a robust vehicle to insert and remove easily without tools or the risk of bending the IC pins. This is only one of our large range of prototyping modules - check the full range today!