Orbit Tool

Drag with the orbit tool to rotate around an object in a 3D view.

You can always access this tool by holding down Ctrl and dragging the Middle Mouse Button (MMB), or you can just pick the tool from the tools panel.

The arrow keys also let you orbit around a model, as does dragging on the view control that looks like this:

NB: If you orbit in any view other than Perspective the view snaps back to it's original orientation when you let go of the button. This ensures that the Front view always shows the exact front of your model and so on.