Private Notes plugin for WordPress provides registered users on a site with the ability to send messages to one another.
Cloud-O-Google Queries is a filter to display keyword clouds based on search engine queries.
WP Group Restriction is a WordPress plugin that enhances the current user role feature, by allowing administrators to manage access to content (i.e., pages).
The Organize Series plugin helps with the organization and presentation of articles/posts you write as part of a series.
Slickr Gallery is a bandwidth-friendly AJAX photo gallery plugin for WordPress. It allows you to pull your Flickr-hosted photos into a gallery section of your blog, as well as easily add Flickr-hosted images to your posts and pages. It is especially useful for people with many Flickr albums/photos. Slickr Gallery needs a Lightbox plugin to work properly.