RubyAMF is an Open Source, MIT-licensed (with one exception, as explained in a moment) Flash Remoting gateway for Ruby on Rails. It focuses on speed, and it integrates directly with RESTful Rails controllers via render :amf. Because of this, RubyAMF can be used as a more efficient wire format for sending information between Flex and Rails. (Of course, any time you see a claim that something is more efficient, you should do your own benchmarks or profiling. As such, I’m not going to do it for you here.)
RubyAMF also features a standalone implementation that doesn’t depend on Rails—it’s called RubyAMF and not RailsAMF. But because this book is about Rails in particular ...