Google Blog has a nice article on Comment Spam. If you're a blogger (or a blog reader), you're painfully familiar with people who try to raise their own websites' search engine rankings by submitting linked blog comments like "Visit my discount pharmaceuticals site." This is called comment spam, we don't like it either, and we've been testing a new tag that blocks it. From now on, when Google sees the attribute (rel="nofollow") on hyperlinks, those links won't get any credit when we rank websites in our search results. This isn't a negative vote for the site where the comment was posted; it's just a way to make sure that spammers get no benefit from abusing public areas like blog comments, trackbacks, and referrer lists.

Any link that a user can create on your site automatically gets a new "nofollow" attribute. So if a blog spammer previously added a comment like

Visit my <a href="http://www.example.com/">discount pharmaceuticals</a> site.

That comment would be transformed to

Visit my <a href="http://www.example.com/" rel="nofollow">discount pharmaceuticals</a> site.

Any piece of software that allows others to add links to an author's site (including guestbooks, visitor stats, or referrer lists) can use this attribute. Google is working primarily with blog software makers for now because blogs are such a common target.

Here's a sample full code listing that bloggers can use to prevent comment spam in their blogspot blogs.

<p class="comment-data">
<script language="JavaScript">
function PostComment(frm){
var name = "";
var email = "";
var url = "";

if(frm.name.value=="") {
name = "Anonymous";
} else {
name = frm.name.value;
}

frm.postBody.value = "<a rel=\"nofollow\" href=\"" + frm.url.value + "\" title=\"" + frm.email.value + "\">" + name + "</a> wrote: " + frm.Comment.value;
frm.submit();
}
</script>

<form name="CommentForm" action="http://www.blogger.com/add-comment.do" method="post">
Name: <input name="name" type="text" size="30"> <br>
Email: <input name="email" type="text" size="40"> <br>
URL: <input name="url" type="text" size="35"> <br>
<textarea name="Comment" rows="10" cols="40"></textarea>
<br>
<input type="button" onClick="PostComment(document.CommentForm)" value="Post your comment">
<input type="hidden" name="blogID" value="7687465">
<input type="hidden" name="postID" value="<$BlogItemNumber$>">
<input type="hidden" name="anonymous" value="y">
<input type="hidden" name="postBody" value="">
</form>