In Visual Studio when you click the Auto-hide thumbtack looking icon the window will collapse into a band that has buttons for each hidden window for the docking area.
I've seen this in other apps so I assume its a control that I haven't found or figured out how to use yet.
The .NET Framework does not include a control that has that behavior out-of-the-box (I believe it's called a dockable pane control). However, numerous third parties offer controls that mimic it quite well:
http://www.codejock.com/products/dockingpane/
http://www.infragistics.com/dotnet/netadvantage/winforms/windockmanager.aspx