The most famous free online encyclopedia did it share to stop spammers a few days ago. Wikipedia inserted the 'not follow' tag in order for search engines to skip outbound links.

Wikipedia said stop to spammer abuse of its concept. According to Computer Active the not-follow tag will significantly improve encyclopedias chances of remaining spam-free:

"The site has banned search engines from following links from Wikipedia to external sites. Previously, it was possible for spammers to create pages on the encyclopedia with links to their products. Because of the way most search engines work, the fact that the link came from Wikipedia would give it an added authority, and push it up the search engine's ranking.
From now on, all pages on the site will have a special tag added to their links which will instruct search engines that the links are not to be followed. The fact that anyone can create and edit pages on the site has made it an attractive target for spammers."

The tag is also useful for all of us bloggers. Use it to stop spam-commenters spamming with links. They will be able to post their links, but search engines will not give it any significance or rating based on the link alone, until you get round to deleting it.