Git

Git is a free, open source version control system. It allows you to not only keep separate versions of your work, it allows you to revert to an old version of your work should you make a change and everything breaks. This is particularly important when doing custom work, such as theming or building a custom module, or when upgrading modules, as relatively minor things can sometimes cause everything to go haywire in your Drupal installation. Check out Chapter 4 for more info on using and installing Git in your development environment.