Sites like Wikipedia, Flickr, Google Knol use the rel=nofollow tag for all outbound links thus denying Google Juice to all external websites.

While most big sites use the nofollow tag to prevent spam in the system, Alan Carr is not too happy about this and has created a WordPress plugin that will automatically add rel=nofollow attribute to all links in your blog posts that point to the above sites.

This looks like an interesting tit-for-tat approach but my advice is not to u