Foundation is a toolbox for Sketch that helps designers quickly compose robust user interfaces using flexible components and interchangeable libraries. You can download it for free on Github here.
Foundation decouples basic libraries from your main design document to make it easier to adapt the design system to different branding or visual styles. Rather than rebuilding a button component for every new project, use the "Button" element from Foundation and skin it to your styling via the overrides. This means designers can focus on creating experiences, not components. You can also leverage Sketch's file versioning and history to track changes specific to each color, type, and element library, too.
Since each library governs an aspect of the design system, it's easy to swap out parts with project-specific design elements. If you need a custom set of brand colors, just make a new document called "brand_colors.sketch" and add some swatches that are 200px x 100px. Save it as a library and Sketch will automatically find those and allow you to swap them into the Foundation design elements. You can even add those swatches to your sketch file as a symbol, instead of using a library, to save some steps. Overall, this reduces large-file redundancies as designers only add to the existing libraries instead of cloning them.
Most UI elements are made of basic shapes that mask an imported color swatch. Any text references an external type library, and icons are from the icon library. This approach offers 2 key advantages:
Foundation lets designers spin up an application design without having to create a single symbol in their design file. Rather than green-fielding new components each time, designers can simply import an element, modify it's properties to their needs, and get to designing. Imported symbols are nice because they won't clone themselves accidentally when you duplicate or copy / paste an artboard (if it's happened to you once you know how scary it is to untangle repeated symbol parents in Sketch).
Alternatively, if you don't want to copy / paste symbols around your document you can still import an element from the library, change all of the overridden properties to suite your app, then save THAT as a symbol in your document. This creates a "preset" of sorts that you can reference any time in your design and you won't need to bother with changing the overrides again. You can also "lock" specific attributes of this symbol so you don't accidentally change the text-style or background color.
Foundation offers these amazing features, too:
Unlike other design kits on the market, Foundation won't give you a skeleton banking app out of the box. It isn't designed to be an instant Dribble portfolio, either. This toolbox is meant to give designers an awesome grab-bag of parts to start building their own interfaces - without all the tedium of making components and symbols every time.
Foundation is also a fantastic addition to any design system, offering governance around key elements like color, type, and iconography that can be iterated on without breaking the consuming design components. It's also free, because why not. Download it now!