The AdaOS is an Open Source
effort to write an entire operating system
in the Ada
From The AdaOS homepage.
Have you ever been in a situation where your computer has really let you down? The dreaded Blue Screen Of Death? The sudden, unexpected, random, spurious "This program has performed an illegal operation and will be shut down"? The crash just when you truly, madly, deeply don't need a crash? The silent, mysterious, and, above all, utterly infuriating system reset? Have you ever wailed in the depths of the night "There Must Be a Better Way of Doing It!"?
Well, that's what this project is about. It's about designing an operating system that does it better, in terms of robustness and reliability, in terms of efficiency and resource parsimony, in terms of proper security facilities, and in terms of good application design and true user-friendliness.