Serial Console?
Our headless systems don't have VGA output as well as keyboard and mouse connectors. They rely on the serial port for software installation. Unfortunately, the current Debian Linux CD-ROM installers don't seem to support serial console out-of-the box.
Most of our systems also have serial console redirection support in the BIOS. It is a convenient way to check a pfSense firewall in a datacenter with just a serial-USB adapter and hyperterminal.