The RS-232 serial port on network switches and routers (and other devices) remains a staple even on modern equipment; if you've just hosed yourself out of the router with a bad config edit and you can't telnet back in, the console port is your only hope.
But, in this day and age, all Macs and most if not all PCs come with NO serial port anymore; and there are zillions of USB adapters, but some are only for PDAs, and others need drivers that aren't supported anymore, or conflict with something else, or whatever...and then, even if you've got the hardware all set to go, how do you actually make the connection? And even if you figure that out, what if you've got a Mac?
This is one of those things which is so basic that it's almost impossible to find out how to do it, because no one's written it down, because everyone already knows how. Or something. Whatever the case, I had a hell of a time figuring it out. So here's a quick-n-dirty recipe for doing it on a Mac (in this case, a newish Intel-based MacBook). This is just one viable combination of hardware and software, that worked for me.
The first thing you need is a USB to serial adapter. As mentioned earlier, there are lots of different ones made by various manufacturers. Many people have reported good results on a Mac with the Keyspan brand; I happened to pick this particular IOGEAR one, largely based on the low cost, but also on an Amazon customer review that said it worked for this specific application. But I probably wouldn't have needed to hunt down the Sourceforge drivers if I'd gone with Keyspan -- who knows.
IOGEAR USB to Serial Adapter, model number GUC232A
Don't even bother to install the drivers on the CD that comes with the adapter. They don't work (or at least they didn't work on my MacBook; they may work on a PPC). Download and install this instead:
http://sourceforge.net/projects/osx-pl2303/
and reboot as prompted.
Plug in the adapter to a USB port on your Mac, and plug in a female-to-female serial cable (these come with HP and Cisco gear, and probably all the others too) from the serial "side" of the adapter to the Console port on your switch/router.
Open the Terminal and run this (NOTE: the numbers after "PL2303" may be different on your system; hit the [tab] key to auto-complete the rest of the device name):
screen /dev/tty.PL2303-0000101D 9600
Hit the return key twice. You should see text in the window, then, depending on the device, a prompt to hit return some more, or you may just be at the device's OS prompt. If you don't see anything or if it's garbled, try restarting the network device, and hit the return key every few seconds until you get what you would normally see in a telnet session to that device.
When you're finished with your session, log out, then unplug the USB cable from your Mac. This will terminate the screen session. It will also trigger a warning about removing the USB device while an application is still active; you can safely ignore that.
One alternative to Terminal/screen is the ZTerm program. Make sure that the USB adapter is plugged in before you launch ZTerm, or it won't be listed as a possible port. Hold down Shift while launching ZTerm and select whichever port has "PL2303" in the name.
References:
A Cisco Blog post (a user comment led me to the Sourceforge drivers)