s2Member is an extremely powerful ( and free ) membership management system for WordPress®. The s2Member plugin ( free ) integrates seamlessly with PayPal® ( very easy ), and fully supports recurring billing, with the ability to track affiliate commissions on a recurring basis. s2Member supports custom Pages for registration ( including Custom Registration Fields ), account access, and a whole lot more.
s2Member Pro is an add-on module for the free version of s2Member. s2Member Pro adds PayPal® Pro integration, PayPal® Pro Form Generators, Authorize.Net® AIM/ARB integration ( also w/ Pro Forms ), AliPay® ( China ), Google® Checkout integration, ClickBank® integration, ccBill® ( for adult-oriented sites ), advanced User Import/Export tools, Coupon Codes, a Pro Login Widget ( plus Profile Summary ) and more.