Redox OS subscribes to a micro-kernel design but part of what makes it so interesting is that it's written in the Rust programming language. Most features are implemented in Rust for Redox OS and there's an optional original GUI, Newlib for C programs, drivers are run from user-space, and there's work underway in supporting the ZFS file-system. Common Unix commands are supported by Redox.
Redox OS is an MIT-licensed open-source project. The current installer for Redox OS weighs in at just 26MB for the ISO.
Those wanting to learn more about this exciting new microkernel-designed OS project can find out all the details via Redox-OS.org. The Rust code for Redox is hosted on GitHub.