Tilt and Shift lenses are special lenses for landscape and architecture photography. I’m e.g. using Canon TS-E 17mm f/4 L and Canon TS-E 24mm f/3.5 L II lenses. The T/S lenses allow adjusting the plane of focus quite freely. The plane of focus does not need to aligned with the sensor plane, you can set it even perpendicular to the sensor plane. This can be used e.g. to align the plane of focus with the ground level. This is a good tutorial about T/S lenses.
All T/S lenses are manual focus lenses, and if you use the tilt feature, the focusing (e.g. aligning focus plane with ground) can be tricky. There are some pre-calculated tables (link to pdf table) that tell you the needed tilt based on camera height and focal length. You can also focus the T/S lenses without any aids, just by iteratively adjusting the lens tilt and focus. But that can be time consuming.
Guillaume Sébire, a friend of mine, has made Tilt Calculator application to iPhone (link to iTunes) to determine the needed tilt. You input the focal length, camera height to the ground (or to be precise the hinge line distance, see the last image), and camera body tilt to the application (I’m not using focus distance input). Based on this information the application calculates the needed tilt and focus distance. It has even automatic body tilt measurement: you just align your iPhone to your camera body (e.g. put the iPhone against the LCD display), and application measures the body tilt automatically (see image below).