I’ve been playing with Media Wiki on my Linux web server. This is the Wiki engine software used to power Wikipedia.
I noticed that one of the options was to turn on – rel=”nofollow” on external urls.
What this does is provide a means of preventing an external web site link from giving a site a boost in the search engine ratings.
It doesn’t prevent anyone from clicking on the link, or have a negative effect against a site, it just means that the search engines will ignore the link from a rating point of view.
Google supports this option as well as other search engines such as Yahoo.
This is therefore recommended for anywhere that a url can be edited or submitted by a user. Typically this includes Wikis, forums and blog comments.
For more information see: Google Blogspot – preventing comment
If you view the source of this post, you will notice that the above link includes the rel=nofollow option.