jQuery EasyUI framework is a complete UI library for creating web pages fast and easily.
Every feature is offered as separate plugins which include drag-drop, pagination, accordion, tabs, panel, menu, form + validation, modalbox, datagrid and much more.
It has support for theming and comes with a default and grey theme. Also, it is multilingual, already supports multiple languages and new ones can be added quickly.
jQuery EasyUI also has various extensions like portal or an editable datagrid.
The library is very well-documented, comes with examples and updated regularly.