Cool.io is an alternative to EventMachine, albeit one which using Ruby’s own native I/O primitives rather than reinventing them, and does as much as possible in Ruby instead of C, which should make it easier for interested contributors to hack on.
As someone who has been using Reactor-style event frameworks for a very long time, but who has never liked the EventMachine API, I’m very pleased to hear of this. Nice to see the Rev project get some new live breathed into it.
Also, yay! Another project I can knock off of my list of things to do.