Given an XML Schema (not a DTD) -- generate XML instances that are valid against the given schema.
This is very useful in creating tests suites, as it allows production of "positive" examples. For negative example generation, I am not familiar with any available tool other than untidy (which can only cause "not well formed" problems but also gives well formed examples by manipulating rules that give different representation to what is syntactically/semantically the same thing, such as adding/removing whitespaces).
Here is a list of available tools for this task:
* xml spy
* XML-XIG: XML Instance Generator (pronounced XML-ZIG)
* Stylus Studio
Anyone aware of alternative (better) tools?