Many Hub contributors have invested in proprietary XML editing software such as XMetaL or Oxygen, but if you haven’t yet done so you might be interested in a free program called XML Copy Editor. It comes in Linux and Windows versions and makes it easy to validate EAD files. It’s also easy to associate style sheets such as the Hub’s so that you can get an impression of how your file will appear in a browser (which makes it a lot easier to proof-read). One feature I particularly liked was the ability to choose which browser to use for this: this is a failing in XMLSpy, which defaults to Internet Explorer (which then always crashes on my machine). I’ve never worked out how to change it to use Firefox (or any other browser), but it was easy to do that with XML Copy Editor.
Of course you don’t get all the functionality of the proprietary systems, but you are prompted as to which tags are valid at any point in the file, and there’s a handy special-character insertion menu, so if you don’t mind working with EAD tags visible, this looks like a good alternative to the paid-for options.