Trackbackers is a plugin that allows you to link back to the domains that send the most trackbacks to you, much like top commenters. It might be a good idea to run some kind of trackback validation or Akismet with this plugin.
High Keyword density Plugin checks for the user-agent and removes certain common words from the post text accordingly. So when Googlebot visits your page, it wonâ€™t be seeing these words, while your readers can still read your meaningful sentences. I am not sure of the viability or the effectiveness of this plugin but it might be on the border of being Google un-friendly. Please use at your own risk [EDIT] We choose not to promote this plugin. Please see comments below.
AJAX Edit Comments 1.1 allows admin and users to edit comments from a post. Users see a countdown timer when editing a comment. Admin can now also edit, delete, and mark as spam from a post without having to go to the admin panel. In other words, any user that leaves a comment on your blog can edit their comments.
Date Exclusion Plugin gives the user control over where and when the timestamp should be displayed. You can exclude dates for all the posts, or only on posts older than a few weeks or months and this time span is customizable.
Google Maps Made Simple Plugin allows a user to add Google Maps to a WordPress post and allow the user to set the zoom level and the width and the height of the map.