Another brand new device is the Logic Level Converter Module. Many of the most interesting sensors and devices are only available in 3.3V or even lower voltage versions these days, which can be a problem if you want to connect them to a 5V microcontroller such as an Arduino. This module easily connects different logic voltage levels together for bi-directional communication on up to 4 channels, allowing you to use low-voltage sensors with a 5V microcontroller.
The module acts as a "bridge", linking high-voltage and low-voltage parts of your project together so they can happily talk to each other. See how it can be used in the Logic Level Converter Module Quickstart Guide. Best of all they're dirt-cheap at only $6.95, so it's worth having a spare around in your parts drawer just for those frustrating times when you have an Arduino in one hand and a 3.3V device such as a GPS module in the other, and want to convince them to be friends and play nicely together.