Rapidly creating structure with the Diagramming tool
This tool is for rapidly creating new objects on the canvas. This tool is especially fun if you have automatic layout turned on. 
If you have nothing selected, click any empty spot on the canvas to create a new generic shape there and select it.
Once you have a shape selected, click any empty spot on the canvas to create a new shape connected to the original one. Hold Option to create a sibling (connected to the same parent as the selected shape) instead. Hold Shift to add an aunt (essentially reversing the connection line of the new object), or hold Command to create a completely disconnected shape.
Click an existing shape to select it, so that you can use it as a starting point for creating more connected shapes.
Once you get the hang of the modifier keys, you can create really complex diagrams with just a few clicks.