The xmld20 project is my attempt to come up with XML Schemas for d20 data, to allow both human-readable data for d20 gaming (with a focus on Dungeons & Dragons), as well as allowing for machine processing. The project will attempt to follow along with the design of d20-xml (found over at Yahoo! Games Groups).
Update: since the d20xml project is dead, I'm not really trying to follow anyone else. The E-tools format would be good to be able to support, and the RPGXplorer system might have a structure worth following enough to port to and from, but as of now (2/8/2007), I'm flyin' solo in my design aims.