Which hardware sensors are supported by Android? Which hardware sensors are supported by Android? Are there any imaginable sensors that may be planned to be supported in the future? 

For every sensor:


How is the sensor useful?
Which devices currently have it? (Only for new sensors)
What do notable apps use the sensor?


Here is a list of currently supported hardware sensors:

Environment Sensors


ambient humidity
illuminance
ambient pressure
ambient temperature 
Device temperature


Motion sensors


accelerometer
gyroscope


Position sensors


geomagnetic field sensor
the proximity sensor



  The orientation sensor is software-based and derives its data from the
  accelerometer and the geomagnetic field sensor.
AFAIK, these are sensors which Android supports:

- Accelerometer

Measures amount of acceleration which the gravity applies to the device, in 3 dimension. Therefore, the device can detect exactly in which position against the earth it is. Its usefulness is very wide, but mostly in games.
Almost all of Android phones have this sensor.

- Gyro

This one is very related to accelerometer and according to Wikipedia:
"The integration of the gyroscope has allowed for more accurate recognition of movement within a 3D space than the previous lone accelerometer within a number of smartphones. Gyroscopes in consumer electronics are usually called Accelerometers (Acceleration sensors) and come in 3-axis, 6-axis, etc."
Not all of Android phones have this sensor, but most of newer ones have.

Full List of Android phones with Gyro sensor

- Compass

This one almost works like a normal real compass, but very very tiny! It allows device to detect in which position it is against the poles of earth. So, apps using this sensor can show you the angle which you have hold the phone comparing to the North, South, East or West. It's main usage is in apps to find your way, in companion with GPS, or for example finding Qibla for Muslims, or any other interested direction which you may need.

List of Android phones with Compass

- Orientation

I Think this one is not a real sensor, but a side usage of Accelerometer. Orientation detection is most used for Android UI and most apps to detect portrait or landscape position of the phone, to rotate the UI accordingly. Almost all of Android phones have this feature.

- Pressure (Barometer)

This one is relatively a new sensor, which detects pressure of air in area which device is in, just like a barometer. It's main usage is to detect altitude of current location of device from sea level. This helps GPS to be faster and more accurate because one of 3 main values (Latitude, Longitude, and Altitude) that are detected by GPS, can be faster detected by barometer. So, GPS would work faster in companion with barometer sensor.

List of Android phones with Barometer sensor

- Proximity

This sensor is mostly located beside earphone in top of front face of the phone. It can be seen easily if you put the phone under bright light (unlike above sensors which are inside the phone and not visible). Its main usage is to turn-off screen, when you are calling and phone is near your head. Android turns off the screen, so it prevents accidental touch of screen when you are not supposed to, and also, it reduces battery consumption in calls. Some apps also use this sensor to control phone by your hand gestures, because this sensor has 2 state, On and Off. On for when something is near it (about 1 cm) and Off for when nothing is near. Most Android phones have this sensor.

List of Android phones with Proximity sensor

- Ambient Light

This sensor, as obvious from its name, measures ambient light in environment where the phone is in. Its value is a number between 0..255 (darkest to brightest) and is mainly used by Android internal system to automatically adjust brightness of screen to be properly readable under bright light of the sun, and also in a dark room. Not all Android phones have this sensor, and no usage except adjustment of screen brightness can be considered for it. I couldn't find a list of phones with this sensor, but almost all if high-end Android phones have this sensor. Its favorite location to sit down in your phone, if ever exist, is near earphone at top of front panel, and looks like a tiny and dark front camera placeholder.