While building a project with a 4×7-segment display, I was irritated by how many pins needed to be used just for the display. I barely had enough to be able to complete the project.
So I looked into methods for controlling an LED matrix with fewer pins. I learned about Charlieplexing, which seems interesting but non-trivial. Then I found the MAX7221 IC, and an Arduino library to interface with it. Perfect. I bought some to try out and they’re very easy to use. Though the display I was using would not run on USB power — I had to connect it to 7.5V from my bench supply.