Strictly not a framework but instead a tool that will allow you to develop your own framework built upon the atomic design approach developed by Brad Frost. Once you have finished a project with pattern lab you will have your very own site framework to continue to build new sections and features on your site. If you like the idea you should also pre-order Brad's book all about Atomic Design.
My personal favourite of the full featured frameworks, Foundation is now up to version 5. It has had a few forks in the road along the way too, notably Foundation for Apps(http://foundation.zurb.com/apps/) (built entirely upon Flexbox and awesomeness) and Ink — a responsive email template(http://zurb.com/ink/) (that even works in Outlook). Foundation can be customised and works on Sass to allow you to change anything that comes out of the box.
Bootstrap was found to be the [most popular framework on the web](http://reports.quickleft.com/css) holding almost an 80% share when it comes to detectable frameworks (check the article for clarification on the numbers). Initially developed using only less it now supports Sass as well.
A responsive framework based upon the newly released Google Material Design principals.
During a recent review 960.gs ranked as one of the more popular frameworks, however it has since been surpassed by Unsemantic which is effectively the responsive version of 960.
This framework only came out of the woodwork after doing a lot of searching on the web. It is not as popular as other frameworks like Foundation or Bootstrap, but is a neat little lightweight solution.
Gumby Framework is a flexible, responsive CSS Framework, Powered by SASS. Create rapid and logical page layout and app prototypes with a flexible and responsive grid system and UI kit.
Harry Roberts (CSS Wizardry) is responsible for this neat little framework. It was created to allow serious developers to use it as more of a set of guidelines that you could plug in your requirements and it would provide the results you needed to develop your design. This does not provide tables, layouts or anything for you.... that's a bootstrap kind of thing. Since I previously covered Inuit it has moved to a new home on GitHub and Harry is preparing some pretty awesome things for it.
Susy is built by Eric Suzanne and is one of my favourite non-framework frameworks. Essentially it's a calculator for the design that you want to build. As it states — "Your markup, your design, your opinions | our math."
It's okay, you're not seeing double. Suzi (with a z and an i) is a mature, feature-rich, responsive Sass and Grunt UI framework built and managed by Matt Stow.
Developed by Joni Korpi and provides 4 keys requirements for your grid. Folding columns, elastic gutters, baseline grid and a lovely overlay to preview your grid. The owner demands that you don't take this as it is, you must pull it apart and take what you need and make them fit how you work. Spread is another tool based upon Golden Grid System that has been implemented using Compass and SASS.
Centurion is a HTML5 / CSS 3 based framework for rapid prototyping. It comes out of the box, or the Git with pre-built features that help you build sites faster for Desktop and Mobile.
Skeleton is a small collection of CSS files that can help you rapidly develop sites that look beautiful at any size, be it a 17" laptop screen or an iPhone. Skeleton is built on three core principles — responsive grid down to mobile, fast to start, style agnostic.