It's when you post something that's really controversial or attacking a big name blogger, for example, so other bloggers / diggers / etc will link to it and show it to people.
www.prmaximizer.net can helps you to automatically find the sites but you need to manually contact to the webmasters.
yes agree, article marketing, linking, forum and community marketing, and good email marketing to targeted visitors is also help a lot.
For more information about link bait - www.seobook.com/archives/001936.shtml Link bait method is part of the free traffic.