I've made a new plugin to allow users to subscribe by email to different categories. Respect given to Scott Merrill for his work on the subscribe2 plugin, which is used within this plugin.
This plugin provides an advanced subscription management system for WordPress blogs. Visitors can subscribe and unsubscribe through a web-based form. They also can manage their subscription (add or delete category(ies) to which they want to be subscribed). All requests require opt-in confirmation by email.
AJAX is used for receiving datas about accounts (validity of email address, the categories which the subscriber has chosen, etc.). Therefore there is no need for page refresh. If the entered email does not exist in the database, the system offers to subscribe to the list. Otherwise we enter the account where we can choose from what categories we would like to receive posts.
More information you can find on http://www.e3internet.com/greenhouse/kirill/advanced-mailer-using-ajax/12/2005/
All feedback welcome