This is an old post, preserved for reference.
The products and services mentioned within are no longer available.
Here's the first Arduino based project, or at least developed on an Arduino.
RWAP and bought one. They are very good replicas, the only noticeable difference I could see is the © date at the top, 1980 vs 2008.
Arduino USB Library seems to be in good order, a port of V-USB, and little is required to get a USB keyboard up an running. I built the circuit on a propto shields and added some keys on breadboard. The major stumbling block for me was most of the sample circuits on the net used pins D2 and D3, where as the config file supplied used D2 and D4. Once I'd changed that, my little 6 key keyboard was up and running. I also tested it with the new membrane and that was working as well.
I was probably watching the same thing the last time I used the original ZX81, just on a slightly smaller TV.
2022 Note: this was a very primitive version of the USB keyboards I produced later on, with custom PCBs and custom firemware. I have left this post here for historic reasons, but I have got rid of all of the other USB keyboard posts as I think there time is over, and I don't want to get any more requests for them.
2023 Note: Still getting requests, so I have listed some of the newer custom ZX81 USB keyboard controllers on my SellMyRetro store.
Two versions, one for use within a ZX81 case, and one to use a ZX81 membrane standalone for your own project not in a ZX81 case.