How to open XML files?

There are several XML editors available at VersionTracker or MacUpdate.

Additionally, you can use the CLI tool plutil, the Developer tool, Property List Editor (requires installing XCode Tools or using Pacifist to extract it from the appropriate pkg), or, if the files not binary, drop it on TextEdit.

Follow-up: Article in Jul 07 MacTech states:

The "textutil" tool can convert and manipulate many different formats of text documents, such as:
txt, html, MS "doc", MS XML, rtf, and webarchive.

You might look at that.

XML files are just plain text. You can open them with any text editor.

Nnot all are plain text. Some, plists for example, are binary.

Yes, but a plist file isn't a true xml file, is it? it's a plist file, which can be binary or xml text.

You can convert binary plist files to xml using:
plutil -convert xml1 myplistfile.plist

That's why I mentioned plutil and textutil. Hopefully, the OP will find a solution amongst the recommendations. Have a joyous 4th.