I'm looking for an open source tool to generate diagrams from XML Schema documents, similar to the Logical Model View in oXygen or the diagrams in XML Spy's Schema Editor.
Check out StarUML I have used it to reverse engineer from source code and it worked pretty well.
Eclipse does basically what Altova's product does as far as the link you shared. Just download/update eclipse with the web and xml plug-ins/components. It has both visual and graphical views. I have found it to be quick for development, but am not sure if you can print the views easily.
Nice wiki entry showing what I am talking about: http://wiki.eclipse.org/index.php/Introduction_to_the_XSD_Editor
This is more of a hint than an answer.
Using ruby and graphviz one can turn XML documents into visual counterparts, as this snippets shows. However, effort must be put in to write a generic script that turns any XSD into colorful graphviz charts, but I imagine, a lot of people would be happy about a single (ruby) script which performs such a task.
Another - at least free - solution could be the community edition of liquid xml studio.
hyperModel is an Eclipse plug-in that can reverse engineer UML diagrams from XML schema.
The nice thing about Oxygen and XML Spy is their schema centric diagramming capabilities. hyperModel, like startUML is focused on an implementation neutral UML view rather than giving you direct visibility into the XML structure. I don't know of any open source tools that match XML Spy or Oxygen's schema-centric graphical view.
Try XSD Diagram. It's a freeware app that runs Windows (.NET Framework 2.0 required). The diagrams look pretty much like XMLSpy but without color. I ran into one problem on my machine; I could only print the diagram by first opening the Print Preview window and then selecting print from there. I couldn't get it to print from the main window.