Considered by many to be the best framework available at the moment, Bootstrap has a massive user base, and it's still growing every day. Built for developing “responsive, mobile first projects on web”, the framework is designed to be modular and features a wide range of starting templates and layouts for programmers. While it doesn't come with any bundled extras or add-ons, Bootstrap has a dedicated community of developers and third-party plug-ins.
Angular JS may not be quite as popular as Bootstrap, but it does seem to be the current standout framework available. Angular JS features two-way data binding between the Model and View to synchronize info and then lets the user utilize basic HTML to present it. With a crack team of developers provided by the Internet giant Google, Angular JS is one framework that shouldn't be ignored. It especially shines in the world of news providers, with notable deployments that include the Huffington Post website.
Foundation by ZURB
Another popular framework – and possibly the most interesting. This framework has a steep learning curve, but can prove to be very rewarding once you get the hang of it. Once you get the admittedly quirky “handlebars” concept, then things will really start to fly, and the possibilities become endless. Updates are currently being released by the developers at a feverish pace, so you're unlikely to find anything missing. This has all the advanced functionality of Angular (see above) but with a much more compartmentalized methodology. Notable deployments include Groupon and Living Social.
While it would be surprising if you had not heard of any of the above frameworks, hopefully one or two of them can add something to your developer's toolbox on your next big project.