Vera
From WikiTI
Vera is an attempt at writing a community-based operating system for flashable Texas Instruments calculators using the z80 processor. A first forum oriented attempt at this project was abandoned, but the project was picked up again and is now mailing list oriented.
This is an animated screenshot of an interrupt test, showing the splash screen, the console font, and the ability to hook applications into interrupts:
http://timendus.student.utwente.nl/docs/pics/vera_interrupts.gif