I am seriously considering adding a Tilt & Shift lens to my line up. T&S lenses enables one to optically correct the un-natural optical shift that occurs when you tilt the camera upwards to photograph an object/structure - see example:
You can correct this in photoshop, but it is just not the same as properly recording the imagine in the first place.
Tilt movements allow you to obtain a wide depth of field even at the maximum aperture and still keep the entire subject in focus. Shift movements correct the trapezoidal effect seen in pictures taken of tall objects, so as not to distort the subject.
The problem is that a T&S lens is not for the faint of heart, and it is most certainly not a lens for snap-shots, nor is it a lens that you can hand-hold. Canon has 3 lenses in their line-up - all are manual focus lens, and only one is wide angle; the price is not cheap, over $1k - ouch.
I can just see my wife patiently waiting for me to setup the tripod, frame, and focus as I photograph a building during a family trip :-)