Concurrent Ruby
Modern concurrency tools including agents, futures, promises, etc.
...It is small, lean, and loosely coupled, thread-safety, and with backward compatibility. Concurrent Ruby makes one of the strongest thread safety guarantees of any Ruby concurrency library, providing consistent behavior and guarantees on all four of the main Ruby interpreters (MRI/CRuby, JRuby, Rubinius, TruffleRuby).