Interesting tidbit, MoarVM reuses libuv, the core of Node.js. The whole design seems very goal oriented, yet has all the features you'd expect from a modern VM, from Unicode to threads. Looks promising.
It really amuses me that most people know libuv via Node.js. The library is the single greatest innovation of Node.js, having failed to achieve thread safety, and it's now powering both Julia and Rust. For many, many years, there was libevent, and then libev, and finally libuv—the event loop runtime finally hit production grade. If you know anything about Node.js, you should know libuv; javascript is pretty useless without asynchronous event processing.
The event loop has been production grade for a very, very long time, via kqueue, completion ports, and poll/epoll. Plenty of software takes advantage of these interfaces. Having a good cross-platform userspace wrapper is just icing on the cake.
And neovim. https://github.com/neovim/neovim