If you’re a living, moving, breathing person, then the chances are very high that you’ve heard of the newest infatuation in the design world, flat design. If you’re unsure, here’s a quick crash course: it’s flat color. This means there are no (or very subtle) gradients and we are no longer making buttons and other parts of user interfaces look three dimensional.If you aren’t using flat design, today we’ve gathered a boat load of free and premium flat user interface kits. Let’s go right ahead and jump in.