Tilt Shift lenses, which alter perspective and depth of field, are highly specialized and can be quite difficult to master. However, there are a number of creative effects that you can produce with Tilt-Shifts.
A Little More Detail:
Firstly, Tilt-Shifts are very popular for architectural photography. The reason for this is because you control the convergence effect, so that the lines on tall buildings appear parallel, instead of converging to a vanishing point.
Another, more recently popularized, use of Tilt-Shifts is to control the depth of field to creative the illusion of a miniature toy town.
Tilts also enable you to achieve across-the-board sharpness in landscape shots, and are additionally a great tool for where you want to capture multiple shots for the purposes of making one panoramic stitched image.