XML Editing/Viewing Software
For Windows, I found Microsoft's own free XML Notepad to be a great simple to use editor with a nice selection of features. Used it for both reviewing my XML output when developing and editing broken iTunes' libraries. ;)
Requires .net 2.0
I may be old fashioned, but I prefer my text editor.
I use emacs, and it has a fairly decent xml mode.
Most good text editors will have decent syntax hi-lighting and tag matching facilities. Your IDE might already do it (IntelliJ idea does, and I believe Eclipse does as well). Good text editors will be able to deal with huge files, but some text editors may not be able to handle them. How big are we talking about?
I agree that your text editor is probably your best bet. I do know some people who swear by XMLSpy, if you need something that's tailored specifically for dealing with XML files in a visual way. I bet you could find some F/OSS work-alikse but I'm not aware of any.