OptinMonster - OptinMonster helps to convert abandoning website visitors into subscribers and customers.
IntroBar - Welcome and entice visitors from specific sources with an IntroBar.
Subbscribe - Opt-in form to help you grow your web subscriptions
Monarch - Social media sharing plugin for WordPress
UseHoney - Embeddable forms for your website. Design and embed user feedback forms and subscriber forms into your own site in minutes.
WP Optin Wheel - Gamified list-building for WordPress