Just because a domain doesn't have a website, it doesn't mean it's not being used. a lot of companies register domains for internal / email purposes only and never put a website on it.
Damn you beat me to it, but yes I agree, I have several domains just used for mail and other services.
So maybe the domain you want is still being used, also, I registered a domain recently and I added a mail catcher to get wildcard mail, and I received a mass of invoices, love letters, tenders, CV's and the sort for the employees of the previous owners of the domain.
I didn't do this on purpose but the domain was de-registered because of no payment, and just at the right time I registered it. et violas one slip in admin and now allot of people are without email! I still get stuff like the confidential tender we spoke of yesterday, I think the addresses are probably printed on business cards and letterheads and the sort.
So there are financial considerations to think of as well, some domains can reach quite a hefty price because of the effort a company has put into it to tie in with their corporate identity!
Unfortunately I can't "give" or sell them back the domain as I developed a website on it for another company and they have since received mail address and CRM system and even their helpdesk call logging is run on a subdomain.
So valuable lesson, keep your admin in order or let someone else manage your domains!