Privy helps you grow your email list through an extensive library of popups. They have a free plan available, and you can also do a 15 day trial with no traffic limits.
Convertful is an all-in-one tool to turn your visitors into leads and sales through a variety of popup forms and widgets. They offer a free plan.
Beeketing is great for e-commerce stores as they offer a variety of plugins for higher conversions. Some of their plugins are free.
Powr are website plugins to grow your business. They have a free plan available.
Outgrow helps you make quizzes that generate leads for your business.
Jotform is who I use to build online forms. They have a free plan available.
Prove Source is who I use for my website social proof. They have a free plan available.
KingSumo is who I use to grow my email list through viral giveaways. It is free to use the non-Wordpress version (which you can still use on WordPress).