Enable FTP service in OS X Lion, Mountain Lion, Mavericks, Yosemite, El Capitan

Beginning with OS X Lion (client), Apple removed in System Preferences,  the option to activate the FTP server.
Fortunately the daemon is still there, just dormant.

To wake it up launch Terminal and type

sudo -s launchctl load -w /System/Library/LaunchDaemons/ftp.plist

You can disable FTP anytime typing the following command

sudo -s launchctl unload -w /System/Library/LaunchDaemons/ftp.plist