I was using a plugin with WP called Post Notif - basically it notifies (email) subscribers to new posts in the categories of their interest
couple things i really liked about this plug; the developer, Devon, is a great guy, the plugin is well thought out, and it’s completely self contained (no reliance on any 3rd party) - Post Notif is the only plugin i ever found that met my standards
the developer stopped working on it after WP forced the ‘Gooberborg’ editor on us
i asked Jeff Star to take over development and he said he would, but he’s obviously too busy (i think it’s been nearly a year) and so i’m looking for someone else to pick it up
i think this plug has potential but it needs just a little work to fix some bugs and i think it could be even more useful if it allowed people to subscribe to page updates in addition to categories