From JIDE Software:
A powerful yet easy-to-use dockable window solution using pure Java/Swing. It supports full drag-n-drop support to rearrange dockable windows, supports docked, autohidden, floating, hidden, maximized windows. It has many different styles, Office 2003 style, VSNET style, Eclipse style, Aqua style and Xerto style. It can also persist layout and instantly switch layout during run-time.