Google's system for serialising data to network/disk. Much simpler than XML to process and much quicker as well:
http://code.google.com/apis/protocolbuffers/docs/overview.html
In a real world scenario I had to get a Plone 3 site to query a different Plone 2 site for certain data to be returned and processed. In the end I used python's pickle.dumps and base64 encoded the result to send, but this would have been a nicer way to handle it.