Midori, the secret operating system incubation from Microsoft. Midori was based on Singularity (written entirely in M#), but went so much farther. It was twice

Midori, the secret operating system incubation from Microsoft. Midori was based on Singularity (written entirely in M#), but went so much farther. It was twice as fast as Windows, had capability security, Rust-style borrow checking, a full GUI, browser and even Windows compatibility. But there was no future - the OS wars were settled - and now the code languishes inside rather than being open-sourced.