There is a list on Wikipedia.
Radar guns use an optical Doppler effect to measure speed. Their acoustic equivalent is used in medicine, where it's called Doppler ultrasound and used to measure blood flow or other sorts of motion in the body.
Animals that use echolocation can use the Doppler shift to gain information about the motion of their surroundings.
A sonic boom occurs when the Doppler shift shifts a frequency to infinity.
I guess one can continue to concoct scenarios. I wonder whether, when you drop a cat off a cliff, you can hear the pitch of its screaming drop as it accelerates. (It's not all that cruel - cats can usually survive a fall at terminal velocity.)
You could use it to determine which way a whale is swimming if you have two boats, both listening to the same whalesong. You could even use the Doppler shift to gain information about the position of the whale because both the whale's position and its velocity contribute to the observed Doppler shift at any given place. (I don't have any information about this actually being done, but it might be an interesting problem to work out the locus of possible whale locations for an observed Doppler shift.)
I was also curious about whether the Doppler effect gives us information on the motion of the crust that moves during an earthquake. I found this reference which suggests it does.
Here's a similar question which will be helpful to resolve your apparent perception of Doppler effect. Because, Doppler effect is real.
So would it be right to say that when the wave is reflected from B, we can think of it as a source kept on the car B?
No. Because the frequency has already been altered (probably increased) by the moving car and the wind (which can be found using classical velocity addition) and finally, the high frequency sound has been returned to B, which reflects it back - which again gets altered by velocity of B and wind flowing in opposite direction.
Due to the speed of the wind, can we substitute the relative velocity of the source and the receiver in the formula?
You should use the velocity addition formula. Don't forget to account for wind too...
Does the wavelength of sound change if the medium is moving? In particular, if the wind is blowing at the speed of sound in the same direction as sound, what would be it's wavelength?
The speed of sound is a constant at a particular medium. So, if the frequency of sound changes, the wavelength should also change in order for $c$ to be constant. If the wind is blowing the opposite direction to the source of sound at $c$ (no relativistics...), the wavelength is $0$. Because, always keep in mind the sound is pushing & pushing & pushing of the molecules. If there's an opposition pushing it with the same velocity, then its whole energy would be spent trying to oppose wind and finally FAIL...
Best Answer
I would have to see the words in context, but the description "apparent frequency" seems strange to me. The frequency your ear detects is exactly what the most sophisticated scientific instrument would measure. The Doppler shift is real in that the frequency your ear detects is really the sound frequency in your frame.
I would guess "apparent frequency" means that in your frame the frequency of the source is different to the frequency measured in the rest frame of the source. So you could argue that in your frame the frequency of the source "appears" to be different. However I would argue that it doesn't just "appear" to be different, it really is different!
You'll find the same sort of confusion when you start learning special relativity. In your inertial frame an object moving at nearly the speed of light has its length contracted, that is you will measure the moving object to be shorter than someone making the same measurement in the object's rest frame. But there's nothing "apparent" about it: in your frame the object really is shorter.