jQuery UI is a set of themable widgets and interactions, built on top of the jQuery JavaScript Library, that you can use to build highly interactive web applications. The core of the library revolves around different mouse interactions, namely drag and dropping, sorting, selecting, and resizing. 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.

Website: http://ui.jquery.com/