Here are some of the best Membership plugins that help you transform your WordPress site into a Membership site, helping you to monetize your valuable content.

s2Member

Wishlist Member

WP-Member

CNMS WordPress Membership Plugin

Suma Plugin

aMember (has separate WordPress integration plugin)

Memberwing

WP Member Site

I think Wishlist Member, WP-Member and Suma Plugin are the most easy to set up and have the most features. Suma Plugin is the most basic one of these three, and is ideal for just protecting posts on your website and having premium users pay to see this content.

Tutorials

http://www.webbriefcase.net/2009-12/build-a-wordpress-membership-site-with-wishlist-member-review/