<canvas> and SVG. You can design and render simple 3D models on the Web. Zdog is a pseudo-3D engine. Its geometries exist in 3D space, but are rendered as flat shapes.
A tutorial about creating flat shadow effect with CSS3 transform effects and jQuery.
It showcases stories each with a content panel that gives off a nice looking flat shadow. As you scroll through the website, each individual panel dynamically shifts the shadow based on where it is in your window.
Remodal is a flat, responsive, lightweight, easy customizable modal window jQuery plugin with declarative state notation and hash tracking.
DateTime Picker is a responsive flat design jQuery based datetime picker for web & mobile. Users can change values using +/- buttons or type values directly into the textbox by selecting each value fields.
For web, picker can be binded relative to reference element, were it will appear at the bottom of the element. For mobile, the picker can appear as a dialog box covering entire window.