Now that's an interesting question.
This is an old thread so undoubtedly you've found something to do with an unused domain.
I guess that it really depends on the actual domain name itself.
You could use it for email services if your ISP can handle email associated with it
and use it as an email collection/outward delivery mechanism.
Not sure if you could offer it for sale to someone else (you need to check the applicable regulations and laws).
Other than that you could use it for people to navigate to a web page and do something clever (perhaps a site with streaming webcam video of something interesting), a blog, etc
It really is a very open ended question.
can't say I've ever heard of anyone trying to think up a use for an unused domain name.
Hope that helps