I have already searched a little bit aroung google, and haven't found a suitable FOSS alternative for XML editors. I would need some of the nice features XMLSpy has, like graphically analyse a xsd file. It would be nice if it were a cross platform tool.
As an alternative similar to XMLSpy you have in Software Center or Synaptic:
XML Copy Editor is an XML editor focusing on editing document markup languages like DITA, DocBook, WordprocessingML. It features DTD/XML Schema/RELAX NG validation, XSLT, XPath, pretty-printing, syntax highlighting, folding, tag completion/locking, and a spelling/style check.
SiSU is a lightweight markup based, command line oriented, document structuring, publishing and search framework for document collections.
and commong programs like scribus, libreoffice that although not similar will do the work.
Not found in the official Repos of Ubuntu you also have: http://www.syntext.com/products/serna/ (SERNA) which can be installed and is similar to XMLSpy.