Shift Registers
Contributors:
JordanDee
JordanDee 
Resources and Going Further
Well, now you know what piece of hardware you need to use when you're running short on I/O lines. Hopefully this tutorial has helped you connect more inputs to your project.
Looking for another example? Check out this project using shift registers daisy chained with a Teensy:
There is also a good Arduino example in circuit 13 of our SparkFun Inventor's Kit:
If you're curious as to how these devices work, I recommend studying some digital logic, and eventually you will study a device called the flip-flop which is key to how these things work. Also consider looking at a timing diagram in one of the datasheets to get a sense of what happens during the shift. Enjoy exploring!
You can also explore these other tutorials from SparkFun Electronics:
Resistors
A tutorial on all things resistors. What is a resistor, how do they behave in parallel/series, decoding the resistor color codes, and resistor applications.
Capacitors
Learn about all things capacitors. How they're made. How they work. How they look. Types of capacitors. Series/parallel capacitors. Capacitor applications.
Integrated Circuits
An introduction to integrated circuits (ICs). Electronics' ubiquitous black chips. Includes a focus on the variety of IC packages.
How to Read a Schematic
An overview of component circuit symbols, and tips and tricks for better schematic reading. Click here, and become schematic-literate today!