A Tit-For-Tat WordPress Plugin for NoFollow Websites
Sites like Wikipedia, Flickr, Google Knol use the rel=nofollow tag for all outbound links thus denying Google Juice to all external websites.
nofollow
Did Google Penalize Your Website Rankings or PageRank ? Read This
Google has confirmed that they decrease the toolbar PageRank of sites that sell links and pass PageRank without using rel=nofollow.