This sort of project has been done to death, but that doesn't mean I can't put my own original twist on it. Previous ADB to USB conversions I've seen on the interwebs replace one of the two DIN-4 connectors on the ADB keyboard with a USB port. Since i have an absurd amount of vintage macs, i'd like to use this keyboard with old macs and my main battlestation. This means keeping the two ADB ports in the keyboard.
To solve this problem, I found a panel mount mini USB plug. A small amount of drilling in the case is all that was needed to make this mod look at least semi-professional.
The only real expense in this project was a Teensy 2.0 microcontroller. Code was stolen from the tmk_keyboard project.
Installation of the teensy was simple. I used 30 gauge wire to connect the power, ground and data lines on the ADB plug to the teensy. A 1k pullup was placed across the data and power lines. Attaching the teensy to the panel mount plug was done by cutting up a mini-USB cable, soldering wires to the plug, and sticking the mini-USB socket into the teensy.
So far, the keyboard works great. Good enough for another thirty years of use.