Neg, a tiny and un-awesome PEG parser library just reached 1.1.0. This new release brings better error messages in case of unconsumed input and some helper for the translator feature (the thing turning raw parse trees to domain oriented trees).
Sudhindra Rao and Munjal Budhabhatti explain in their "Rocking the Enterprise with Ruby" talk at the RubyKaigi how they are building (and delivering continuously) a system orchestrating Rackspace datacenters
Granted, you can do workflow with state machines, but there's still work to do in order to have a "workflow engine". There are also alternatives. The post "state machine != workflow engine" answers some questions and open up some more. At least, a set of links to Ruby state machine implementations concludes the post.
rufus-decision 1.1 was just released. Decision tables are an elegant / naive way of representing some domain logic. Rufus-decision interprets decision tables (in their CSV representation) and produces an output based on matching fields in the input data.
rufus-tokyo is a ruby gem for handling Tokyo Cabinet databases. It's based on the ruby-ffi gem (mri, jruby and rubinius) and makes cabinets look like hashes. Version 0.1.0 has just been released. Source is on the GitHub.