This is an old post, preserved for reference.
The products and services mentioned within are no longer available.
This is the ZXpand. A great addon for a ZX81 which allows you to easily load files from an SD card.
Minstrels boards, kits and fully assembled units can be bought from my Tindie Store.
2022 Update: At the time of writing, there are still a few Minstrel 2 (ZX80) and Minstrel 3 (ZX81) kits available from The Future Was 8 bit but those are the Final Edition kits, there are unlikely to be any more.