Turned out to be very useful during a recent RMA maintenance window:
sudo launchctl load -F /System/Library/LaunchDaemons/tftp.plist sudo launchctl start com.apple.tftpd
The default tftp file path is /private/tftpboot. [Original source]
You can stop it with:
sudo launchctl unload -F /System/Library/LaunchDaemons/tftp.plist
And, speaking of tftp – there are some interesting projects on GitHub: hooktftp, php-tftpserver and ptftpd.