KnightOS [github] is a third-party Operating System for Texas Instruments z80 calculators. It offers many features over the stock OS, including multitasking and a tree-based filesystem, delivered in a Unix-like environment. KnightOS is written entirely in z80 assembly, with a purpose-built toolchain. Additionally, the KnightOS kernel is standalone, and you can use it as the basis for your own powerful operating systems.
Alternative for this alternative: GlassOS.