The console server may seem quaint, but it's just as useful today as it ever was. The serial console will save the day when Ethernet quits working on a headless server, router or managed switch, or you want to fix other problems with minimal disruption. Just hook up your serial terminal, which could be a special network administrator laptop running Linux and an armada of useful network utilities, with a null-modem DB9 cable and you're in business. What, your laptop has no serial port? (This is common these days.) No problem, just get yourself a USB-to-serial-DB9 adapter.
The serial line is a direct connection to your system, and many devices require no special configuration; it's just there. Simply connect your serial terminal, log in, and away you go. On x86 PCs and inexpensive x86 servers you'll probably have to set up and configure a serial console. A common option is to use Minicom, which is included in all Linux distributions. Minicom functions as both server and client, so it goes on servers and network devices, as well as on the special network administrator laptop.
Minicom is nice, but it's limited. When you're ready to move up to a serial console that includes SSL encryption, multiuser capabilities, access controls, and remote logging, give Conserver a try. Conserver has two parts: a server and a client. You don't necessarily need the client because the server component logs all data so you can replay sessions to find out what went wrong or just keep an eye on activity. For real-time monitoring and system administration, use the Conserver client. The client allows multiple users to log in at the same time, but only one at a time get write access. This is a great feature that can be used for training and group brainstorming on problems. Like any network client, you can connect from any location: directly, over a LAN or the Internet, or via dial-in.
Conserver comes with many Linux distributions, but it may not have been built with the options you want. You can check this by running conserver -V or console -V to see the compile-time options. Visit Conserver.com for downloads, good documentation and mailing lists.