SWX Ruby is a port of SWX (originaly PHP), which claims to be “Ruby’s fastest library for exchanging data with Flash” I haven’t tried it because it doesn’t yet support Flash player 9 but I wouldn’t doubt it’s the fastest as many other methods aren’t too speedy (Although RubyAMF is much quicker lately).
as3yaml, you guessed it, is an Actionscript 3 YAML 1.1 parser and emitter.