JBossXB is part of the JBoss application server, but will also be used in other projects. As far as I understand, it will be moved to a "common" paltform, so that it can be used separately. Currently you can find it in jboss-common-client.jar.
If you have a simple structure like
You basically need a POJO for <foo> and a few lines of code to get things parsed into a list with all Foo-objects.
Alexey has quite some documenation on the wiki page for it.