If you are a longtime Mac user who simply could not bring yourself to throw away your old systems, then you may still have an old Apple Desktop Bus (ADB) keyboard such as the Apple Extended Keyboard II lying around. Despite being an older style, many of these keyboards were quite well-made and if only they had USB connectors instead of their ADB connections they could be put to good use on modern systems. It turns out, this is quite doable.
Recently tech hobbyist Scott Vanderlind found that by adding a small USB controller to the keyboard, he could tap into the device's ADB connection and send it over USB to any modern device, where it works quite well.
The process involves a small amount of soldering, a Teensy USB controller, and a quick flash of the keyboard's firmware to enable the ADB-to-USB conversion of the keyboard's output. Since the Teensy controller is so small, it fits well within the relatively large Apple Extended Keyboard II's chassis, with the only semiawkward result of this hack being the USB cable extending out of a hole in the bottom of the chassis (though one might drill a small hole for it elsewhere).
Apparently with these modifications the keyboard works quite well, with the only hiccup being the need to continually hold the Num Lock key for the number pad to work.
Finally, while this keyboard hack is fun for hobbyists like Scott, it is not the only way to use ADB technology on your Mac. If you are not comfortable taking apart a keyboard you may be fond of, then you can use an ADB-to-USB dongle such as the Griffin 2001-ADB adapter to connect your old keyboard, mouse, and many other ADB peripherals to your new Mac.