CVS, Team Foundation Server, SVN, Git
After years of working on code and making constant changes to site, I was introduced to version control systems like CVS and TFS. As I learnt the importance of the version control systems, I got deeper into the system administration and management of it and then learned the other more modern versions of it like SVN and Git. I have been lately using Github as the primary source control system and have been contributing to various private repositories for private projects.