Builder.io is described as 'Builder provides a very intuitive drag-and-drop web designer for any site. Simply add a small snippet to your site and start creating pages through Builder's easy-to-use and well documented page maker' and is an website in the Development category. There are four alternatives to Builder.io, not only websites but also apps for SaaS, Windows, Linux and Mac. The best alternative is Bootstrap Studio. It's not free, so if you're looking for a free alternative, you could try Layoutit or BuildBootstrap.
Bootstrap Studio is an elegant and very easy to use web design tool for building websites using the renowned Bootstrap framework. With its intuitive, drag-and-drop interface, new users can easily get great designs in just moments.
Layoutit is a drag&drop page layout builder that outputs Bootstrap 3 code. It supports: * Bootstrap grid elements * CSS enhancements of fundamental HTML elements (list, table, form etc.) * components (button group, thumbnail, panel etc.