Ruby has some of the best concurrency support: Fibers and threads are all well-established features. JRuby and Rubinius can handle native threads, no Global Interpreter Lock (GIL) at all.

The downside is most Ruby code bases are reluctant to take advantage of it, or even be thread-safe or fiber-aware, which can make writing apps more challenging than an environment like Node.js where there's established standards and expectations.

Ruby (MRI) is perfectly concurrent already, it just sucks at parallelism.