All major sites like wikipedia, twitter, delicious, stumbleupon adds no follow attributes to external links. Adding no follow attribute to a link in your blog signlas google and other search engines to consider the site but not pass any PR to that. Another way to make best use of nofollow tags is adding nofollow to internal pages that do not have useful content in them to the visitos but still they are needed. Pages like "about us" "privacy policy" "contact us" are...