JEdit

jEdit is a FOSS programmer's editor with a good XML plugin that does validation. It's written in Java, and runs on Windows, Mac OS X and Linux.

To set up full validation of Apertium .dix dictionary files:

1. install jEdit (sudo apt-get install jedit on Ubuntu)
2. start jedit and click Plugins→Plugin Manager, click the Install tab, wait for the list to fill up, and tick XML and click Install
3. click Plugins→XML→Translate Schema using Trang
4. for Input path, choose the file "dix.dtd" in the folder trunk/apertium/apertium/ (it might also be in /usr/local/share/apertium/ if you installed apertium to the standard path, or you could get it from http://apertium.svn.sourceforge.net/viewvc/apertium/trunk/apertium/apertium/dix.dtd)
5. for Input type, select DTD
6. for Output type, select RNG
7. for Output path, select some folder in your home directory and type "dix.rng"; click OK

Now, to get error-checking and validation in a dix file that you've opened, just click Plugins→XML→Set Schema, and open the "dix.rng" file you created in the previous step. Clicking Plugins→Sidekick→Sidekick and the Parse button will give you an updated "tree view" of your XML file. Clicking Plugins→ErrorList→Error List will show you a list of XML validation errors.