Another tool to add to the collection: XML Explorer for playing with XML files. It's often the case when working with XML files that you get given a file that you need to understand (normally due to there being no DTD/Schema, or out of date schemas).
The most common case for me is that I want to visually inspect the file because the tags are not heavily constrained and the relationships are complex.
XML Explorer can handle medium sized files (300mb according to the site) and allows XPath querying - one nice feature being able to have the output from queries to multi-tabs. This makes the process of understanding a particular file much quicker.
Note this is not an editing tools, its just for viewing