Bootflat is an open source flat UI kit based on Twitter’s Bootstrap 3.1 CSS framework. It provides a faster way to create elegant web apps.
Framework7 is a powerful and full featured HTML framework for building iOS7 apps.
Framework7 – HTML framework for iOS7 apps
Typebase.css is a minimal, customizable typography stylesheet. It has both less and sass versions so it can easily be modified.
Typebase.css – A stylesheet for typography
Ionic is a beautiful front-end framework for developing hybrid mobile apps in HTML5. Available on Github.