Well-supported and not loaded with ads, tracking, and upsells

This surprisingly customizable plugin is the way to go if you need popups on your WordPress site. The popup content works like any other post, with a nice preview of what it will actually look like in the edit window, and it can detect embedded form submissions to close the popup automatically. The developer was also very responsive with regards to a pull requests I submitted on GitHub for Ninja Forms 3 support.