Lit CSS Framework - World's smallest responsive CSS framework (398 bytes!)
Bootstrap - Simple and flexible HTML, CSS, and JS for popular UI components and interactions
Bulma - Bulma is an open source CSS framework based on Flexbox and built with Sass. It's 100% responsive, fully modular, and available for free.
mockingbird - Mockingbird helps you you create and share clickable wireframes. Use it to make mockups of your website or application in minutes.
WING - A phone carrier that is actually nice to you.
Purecss - A set of small, responsive CSS modules that you can use in every web project.