LayoutIt! - Interface Builder for CSS Grid and Bootstrap

LayoutIt! is a interface builder for CSS Grid and Bootstrap that wants to be the kick-off for your front-end developments.

www.layoutit.com

HTML Snippets for Twitter Boostrap framework : Bootsnipp.com

A design element gallery for web designers and web developers. Find snippets using HTML, CSS, Javascript, jQuery, and Bootstrap.

bootsnipp.com

Free Bootstrap Themes, Templates, Snippets, and Guides

Start Bootstrap develops free to download, open source Bootstrap 4 themes, templates, and snippets and creates guides and tutorials to help you learn more about designing and developing with Bootstrap.

startbootstrap.com

Bootswatch: Free themes for Bootstrap

Customizable Changes are contained in just two SASS files, enabling further customization and ensuring forward compatibility.

bootswatch.com