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Yagsbook is an XML format specifically designed for the mark up of roleplaying game content. It was originally based on DocBook which does much the same for technical documentation. A Yagsbook document can support multiple types of game systems (allowing d20 and GURPS content to be included side by side for example) and rendered to multiple output formats - currently HTML and PDF are supported.
The purpose of Yagsbook is that document content can be structured in such a way that accessing and manipulating it becomes easy. If a set of rules is written in Microsoft Office, then a chapter listing skills is just a sequence of text, possibly with some fancy headings. It is very difficult to do anything useful with that text - for example pull out a list of skills sorted by group, or filtered according to genre. With Yagsbook this becomes possible.
The best example of Yagsbook is probably the rules for Yags itself. This can be downloaded at http://yags.glendale.org.uk/download.jsp
Note that information is still in the process of being moved across from the old Sourceforge hosted website. Much of the documentation is still on there, and needs to be moved into wiki form.
The source code repository on Sourceforge has been ported across to Subversion. This should provide a bit more flexibility in how the code is managed, plus was an opportunity to re-organise some of the projects.
Due to failure to find an easy way to port across the history information in my 30 seconds of searching (I'm sure there used to be a button to do this), all history has been lost, but it is still available in the CVS tree.