What is the rel=”nofollow” tag?
Simply put the rel=”nofollow” tag can be used when linking to other web sites to tell google and other search engines not to follow that link as part of their crawl. Thats the oversimplified explanation anyway, the reality is that it tells google not to add any weight to that page’s pagerank / ranking because of your link to that site, which it might otherwise do.

How I added rel=”nofollow” to selected sidebar (blogroll) links
If I want to create a link on my sidebar I usually go to ‘write’ and then ‘links’ from with WP Admin. At the bottom of this page there is a subtitle – “Link Relationship (XFN)” and beneath it is a text entry field with the tantalising word ‘rel’ (hmmm I want rel=”nofollow”)…… Unfortunately WP 2.5 won’t let me type anything here. I have to tick a checkbox beneath the text field for anything to appear in it. I guess I need a tickbox called ‘nofollow’. How do I add that?

To do this I downloaded and edited the ‘edit-link-form.php’ file from the \wp-admin folder on my webserver. I inserted the following line of code:

<tr>
  <th scope="row"> <?php _e('following') ?> </th>
  <td>
     <label for="follow">
     <input class="valinp" type="checkbox" name="follow" value="nofollow"
id="nofollow" <?php xfn_check('follow', 'nofollow'); ?> />
     <?php _e('nofollow') ?></label>
  </td>
</tr> 

this was inserted just above

<th scope="row"> <?php _e('identity') ?> </th>

which was on line 174 in my file. Now I have a tick box at the bottom of the write, links page and if I tick it, it makes the link a nofollow link!
** If you try this make sure you take a back up of edit-link-form.php first **

The problem with this is that google sees these archive pages as duplicate content. For example http://www.thinkjim.com/2005/09/ contains a post about a microsoft windows training course, but this post is also a web page in its own right (http://www.thinkjim.com/2005/09/m2276-implementing-a-microsoft-
windows-server-2003-network-infrastructure.html). This counts as duplicate content and can effect your google pagerank.

To get around this i’ve learnt that you need to stop google indexing the archive pages and this can be achieved by adding a line of code to the header.php file – this forms part of your wordpress theme.

<?php if(is_archive()){ ?><meta name="robots" content="noindex, follow"><?php } ?>

If you put the line of code above into your header.php file it will tell google (and other search engines) not to index any pages that are archive pages, but the search engine will look inside archive pages for links to other pages (i.e. posts). If it finds a link it will follow it and index the linked page.