The Open Windows tool is an easy-to-use tool to keep track of your opened windows inside Database Workbench. It can be reached via Windows | Open Windows or Alt+0 (zero).
A typical Open Windows session
As you can see in the above screenshot, by default, all windows are grouped by connection. This can be changed from the Preferences. You can use the context menu to close a connection - including all it's related windows - or select a connection, so that it becomes the active connection.
The Open Windows screen can be docked.
Open windows are also visible on the Windowbar, at the bottom of the main screen.