Steve at Big Mess o’ Wires has been experimenting with USB to ADB converter. He writes:
It’s time for a new progress report on my USB to ADB converter project! My goal is to design a simple PIC32 device that enables USB keyboards and mice to be used with vintage ADB-based Macintosh and Apple IIgs computers. You can read my earlier reports here: 1, 2, 3, 4, and 5. I don’t know when I’ve ever spent so much time investigating the feasibility of a project, without actually doing the project. Hopefully all this preparation will pay off.
More info at Big Mess o’ Wires project page.