http://celluloid.io/

https://github.com/jruby/jruby/wiki/Concurrency-in-jruby

Speaking of -- have you ever heard of evented i/o?

Yes, I wrote a Ruby wrapper to libev before Ry wrote Node, and this coroutine abstraction to get rid of the async gunk, but Ry left that part out:

http://revactor.github.io/philosophy/