Foundation is the second most popular choice amongst designers. Foundation offers developers the finest customization options. It includes an extensive set of templates, and premade code. Every developer can pick their choice at the time of the download. It supports rapid development of projects. It includes a robust grid system, and is superior in many aspects. Foundation lacks in a couple areas, however. Although it has an outstanding grid system, complex customization is needed to tune your project to the best. Modifying codes that Foundation offers is not an easy job for a novice developer. Even if Foundation maintains a mobile first approach, most of the developers prefer Bootstrap as a better in this concern. Most of popular web development companies also prefer Bootstrap.
Another popular choice is Semantic UI. This framework was published under the MIT license. It's very well documented which is a huge pro. It seems easier to learn and use than the other frameworks. It has a grid layout just like the other frameworks. It uses LESS, which extends CSS with dynamic behaviors. It has a very nice implementation of buttons, modals and progress bars. It uses an icon font for many of it's features. It has some very useful extras such as the inverted class. It's open-sourced too which is great. The downsides to Semantic UI are the fact that there is no image slider, thumbnail classes, visibility classes, or SASS.
Pure by Yahoo!
Pure is easy to customize. It has an extremely minimalistic look that is super-easy to customize, since it basically gives designers a foundation on which they can build their design. It's about as lightweight as it gets. It's file size is only 4.5KB. It has a responsive grid that can be customized to your needs. With a solid base built on Normalize.css it has cross-browser compatibility. It works well WITH Bootstrap. (That's right, you can use both!) One major con is that it's not very suitable for beginners. Since Pure CSS only carries a minimum number of styles out of the box, it might not be great for beginners who want a style that looks good out of the box without having to customize it.
UIkit has very clean code and follows well defined conventions. Every aspect of the framework is designed to be modular, this way designers can easily choose which components to add to their stylesheet without risking to damage the overall style. UIKit's rather minimal style can be easily customizable to create an entirely new look to fit the needs of the designer. UIKit has a pretty good and clean style even out of the box without any customization needed. UIKit has some built-in animation features which can be used to animate various components. There are plenty of ready to use themes available from the official website. You can choose the theme that you want to use from the dropdown menu and then download the CSS, LESS or SASS file for that theme to use for the website. A major con is that UIKit is not a very popular framework, especially compared to other options. As such it may be hard to find learning resources other than the official documentation or it may be more likely for development of UIKit to be dropped than for another more popular framework.