Viewpoint

The menu View ‣ Viewpoint lets you align the viewing direction to a specific axis. This can also be done using the Navigation Gizmo or the following hotkeys:

Top

Numpad7

Front

Numpad1

Right

Numpad3

Bottom

Ctrl-Numpad7

Back

Ctrl-Numpad1

Left

Ctrl-Numpad3

The above hotkeys align the view to a global (world) axis. You can also align to a local axis of the selected item by additionally holding Shift. This way, you can for example view any mesh face head-on, no matter how it's oriented. (To get out of this local viewpoint, simply align to a global axis again.)

The view can also be aligned by holding Alt-MMB and dragging the mouse in a certain direction.