Official jQuery User Interface Components Called jQuery UI
On top of the core interactions are built a number of reusable widgets, including accordions, date pickers, dialogs, sliders, table sorters, and tabs. Finally, the library is finished off with a few effects that you can use to liven up your site, specifically magnifier and shadow. The photo gallery and the speed testing application is particular impressive which shows you the power of the UI.
License: MIT License / GPL License