I am slowing working on a few example applications which I hope to release in the next decade. If that's too long for you wait then check out the following XQuery powered apps. MarkMail is a very impressive mailing list archive searching service indexing millions of emails. Atomic Wiki is a wiki style app based on Atom Publishing Protocol and syndication format.
If you're hands on and would like to get started with XQuery you can download the following.
- eXist-db an Open Source XML database.
- MarkLogic provide free community and trial licences for MarkLogic Server. MarkLogic Server comes bundled with an Application Builder to help kickstart development.
- Zorba is an XQuery processor which you can run from the command line. Zorba also has many language bindings.
You can find many XQuery examples on Wikibooks and FunctX XQuery Function Library.