Bitcoin-ruby is a project mainly written in Ruby, it's free.
Ruby implementation of the Bitcoin wire protocol. (De)serialization only - network & state belong elsewhere.
bitcoin-ruby is a library that serializes and deserializes messages in the Bitcoin wire format. Getting the data from (and sending it to) the network, finding peers, validating blocks, building a block chain, etc. are all out of scope.
Specification of the protocol is here: https://en.bitcoin.it/wiki/Protocol_specification
Alternative Ruby libraries that sort-of look at the same area: bitcoin-protocol by altamic and bitcoin-ruby by lian.
This library differs from the others in that it leverages bindata to do the heavy lifting. Results in less, easier-to-read, more robust code.