Google and Comment Spam
Google have published a possible solution to comment spam which involves adding a rel="nofollow" attribute to any third-party links. Seems like a good idea and almost zero cost to implement.
Of course it doesn't just apply to weblogs. Here's how to patch pmwiki to output external links with the new attribute: add the following line to your config.php file:
$UrlLinkFmt = "$LinkText";