Dan Petrovic documented a competitive link hack that can easily been done within Google Webmaster Tools to find all the links pointing to any page on the internet. It is a competitive link hack that seems to work just fine for anyone.
The hack takes advantage of the intermediate link feature within webmaster tools. Without getting too technical, you are tricking Google into thinking your page is a duplicate of a competitors page. Then Google is taking the duplicate version and merging it (on some level) with the competitors page. Since they are merged, Google will show you the links pointing to all the duplicate pages in one report here.
If this indeed works, which is seems to – you can use this to potentially learn about all the links pointing to any of your competitors pages. Someone Google specifically does not want, since they do not provide an open competitive link tool.
I’ve emailed Google for a comment on this and will update the story as soon as I hear back. See also the related story that came out of Petrovic’s research, that duplicating pages from a web site might allow people to “hijack” listings: