How can I use an ADB Apple Keyboard with a modern Mac?
The iMate was the only converter available on the market.

If you do get an iMate, the only thing it will work with these days are keyboards and mice. It internally converts ADB keyboard and mice commands to USB keyboard and mice commands, so it will just make your ADB mouse and keyboard look like a USB mouse and keyboard to the computer, no drivers required.

In ~2001 there was a driver for the iMate that would actually configure it as an ADB bus the computer could see, that way anything with real ADB drivers could use it just like a builtin port. That driver has not been updated in years, and Apple dropped all ADB support from OS X, so at this point USB emulation is the only thing it does. Not that there were many (any?) interesting ADB devices that were supported under Mac OS X anyway.