LibraryThing.com is a (commercial) website that allows users to catalog their own book collections and compare it to what other users have. Picking up on my idea to data mine ISBN numbers out of Wikipedia articles, LibraryThing now for each book presents a list of which (English) Wikipedia articles reference that book. The site's owner Tim Spalding explained how this works in his blog on February 26,
http://www.librarything.com/blog/2007/02/wikipedia-citatons-with-feed.php
The idea is free for taking. There are lots more data to be mined out of Wikipedia (and sister projects) and reused in other contexts. One way to do the data mining is my Extraktor script, http://meta.wikimedia.org/wiki/User:LA2/Extraktor
Note that GFDL must still be respected. But since ISBN extraction and other kinds of cross-referencing don't copy text or images, these actions are not subject to copyright. And since the whole purpose is to link back to Wikipedia, I think there is every reason to encourage this kind of reuse.