As fun as bitbanging might be, hardware implementation of wire-level protocols can be really handy. Hackaday writes about Louis Thiery‘s and Brian Hensley‘s work towards enabling SPI on the Raspberry Pi.
They both show how to get SPI working on the RPi, and then Louis continues to show how to interact with it from Python. This opens up a whole world of cheap peripherals that can be connected to the RPi. Before you go shopping though, a word of caution. The RPi is not very fond of 5V on the GPIO pins. It is recommended that you get a level shifter before you play.