A lot of OpenBSD mentions here, but what about MINIX 3? It's so underrated and ignored :( They made a nice microkernel-based UNIX-like system with a NetBSD userspace.
Also, Redox is switching to a microkernel architecture...
Also, Redox is switching to a microkernel architecture...
re Redox. It's a nice project I've praised before. It's an alpha work-in-progress, changing rapidly, and unclear how much of its design is truly for security. A trimmed OpenBSD is probably a safer bet than it so far given the staggering amount of review that went into it. Rust's features and a microkernel only prevent so many kinds of problems.