A list of free plugins that allows to start a membership with the famous WordPress CMS:
S2 Member fully featured and updated, for extra features (eg. Paypal support) you must buy a pro module
Members it’s basically a role manager and content access plugin that extends WordPress functionality
Pie Register allows to enhance default Wodpress registration, an interesting feature is the Paypal support
WP-Members: contents control access, inline registration and login, registration and members management info directly on your theme, widget, many more
Paid Memberships Pro released under GPLv2 license, many features like a commercial plugin. Allows to create paid membership plans, very flexible