PMPro is a membership plugin for WordPress that makes it easy to charge for access to restricted content on your site.
Customers include dues-collecting professional organizations, e-learning sites, on-demand video sites, and anyone with a WordPress site that wants to charge for access.
Paid Memberships Pro is the only 100% GPL and completely free membership plugin for WordPress. All source code (including extensions) are available for free. Premium support and installation options are available for a fee.