One of my favorite features of the Nikon D800 is the dual-axis virtual horizon which can be monitored through the viewfinder.
Whenever I'm shooting a subject that requires precise alignment, I activate the virtual horizon and then adjust the camera until the horizontal and vertical arrows are centered. This really comes in handy when shooting architectural subjects with an ultra-wide lens. The following images of the interior of the Pachyderm House at the Brookfield Zoo were captured at a 14mm focal length. If the lens wasn't perfectly aligned, it would have been difficult, if not impossible, to correct the distortion of the lines in the image at such a wide focal length.