RetroPie allows you to turn your Raspberry Pi into a retro-gaming machine. It builds upon Raspbian, EmulationStation, RetroArch and many other projects to enable you to play your favourite Arcade, home-console, and classic PC games with the minimum set-up. For power users it also provides a large variety of configuration tools to customise the system as you want.
There is a very detailed guide on how to bring RetroPie to your Raspberry Pi on GitHub.
If you want a How-To-Video, there is one on YouTube by TechTipsta.
For the ones who know what to do, you can download the image right here. Just click on the link for 'Raspberry Pi 0/1' and extract the .gz-file via 7Zip.