AvalonDock is a WPF controls library which can be used to create a docking layout system like that is present in VisualStudio. It supports fly-out panes, floating windows, multiple docking manager in same window, styles and themes and it can host WinForms controls.
This library is the successor of the ‘WPF Docking Library’ that was published on CodeProject quite a while ago (http://www.codeproject.com/KB/WPF/WPFdockinglib.aspx).