"WP eMember - WordPress Membership Plugin!
WordPress eMember is a powerful WordPress Membership
Plugin that can turn your existing or new WordPress site
into a fully featured membership site easily.
WP eMember lets you selectively protect articles by
creating various membership levels (example: Free,
Basic, Premium, Ultimate etc.) and protect the content
(posts, pages, comments etc) of your site. The plugin
manages all the membership management side of things...!"