Collapse is a jQuery plugin for creating progressive disclosure designs.
Basically, Collapse provides a nice, lightweight (600b) solution for expanding and collapsing website content.
What’s the best about progressive disclosure interfaces is that they don’t confuse the users with any information they don’t need at hand.
The additional elements can be shown with a transition, e.g., a sliding motion.
Collapse features a number of options and callbacks (cookie support is notable) thus it’s suitable for creating even the fanciest of web apps.