My personal favorite and weapon of choice.Foundation provides you with a great, flexible grid that can easily be nested. It also focuses on mobile and lets you build sites that are mobile first, then add the stuff for other versions, making the mobile version really optimized and un-cluttered. It also works very well for rapid prototyping, making it a useful tool for more than just designing your site.
Gumby uses a flexible, customizable, 960px wide grid, a width chosen by many web designers. It also uses SASS, aka Synctactically Awesome StyleSheets, an extension of CSS3 that adds nested rules, variables, mixins, selector inheritance, and more. Finally, Gumby is perfect to create clutter-free responsive site, no unnecessary stylesheet on the tablet version of your site.
Built at Twitter, Bootstrap has become the most popular framework and now powers an impressive number of websites. It is probably the most feature-heavy framework in this list and allows you to build pretty much anything you want. Create fixed or fluid grid-based layout becomes easy, as well as making your designs responsive and mobile-friendly.