Brevo (formerly Sendinblue)

Brevo combines email and SMS marketing in a single platform, with a built-in form builder designed specifically for list growth. Forms work as popups, slide-ins, or embeds. The free plan includes unlimited contacts, 300 emails per day, and basic automation workflows. A/B testing is available, and you can set up simple rules to segment your list based on user behavior or form responses.

Ideal for small businesses and agencies managing lists across email and SMS channels. Unlike tools that focus only on email popups, Brevo lets you collect subscribers and immediately message them over both channels from one dashboard. The free tier is substantial enough to grow a meaningful list without hitting paywall limits early.