WordPress Affiliate & Referral is a very useful plugin that extends your WordPress website with a very effective affiliate / referral system. Starting with v3.0.0 the plugin offers the option to pay affiliates via PayPal Pass Payment system.
[*]Easy translations via .po/.mo files
[*]Real-time email notifications for affiliates
[*]Option to pay your affiliates with PayPal MassPay (mass payments)
[*]Unlimited referrals, unlimited affiliates
[*]Statistics (per month, per user)
[*]Custom registration URL option (new)