Since email is one of the most effective marketing channels, it is absolutely critical that every website should have an opt-in mechanism for them to correspond with their visitors. Analyzo has listed the best Wordpress Email opt-in tools for you to choose from.
The Sumome List Builder is a part of a suite of tools developed by Appsumo. List Builder also comes with exit-intent technology similar to OptinMonster. The free version allows a few customizations but for advanced features like A/B Testing you will have to subscribe to their paid plans.
Hellobar is a popular tool that runs across the top of several websites and can trigger several call to actions such as Email Subscribe, Social Following, Sharing etc. Just like most modern opt-in tools, hello bar also lets you A/B test your opt-in forms.
OptinMonster provides a lot of customization options. One of its most powerful features is its Exit Intent technology. It detects when a user is about to leave your website and will cause a popup to appear just before they would leave. Aside from exit-intent, OptinMonster also has trigger options based on time on page, location on the website, manual clicks on images and links, as well as referrer source such as search engine, social, favorite website, etc.
Rapidology integrates with all the most popular email marketing solutions and delivers all the basic email opt-in functionality. It even provides opt-in stats and a good level of customization.
This is a free opt-in plugin from the Wordpress community that provides a high level of customization. It is not as easy to integrate as the other tools, but its free and has good community support. It provides all the standard opt-in features and integrates with a variety of email marketing solutions.