I've thought about this in the past: use arduino to control a faucet so I can turn it on and off hands free. Today I did a search and found this solenoid valve for GE washing machine. I assume if you apply current you connect to hot or cold to the outlet. Has someone done this before? Some input would be appreciated.
I wasn't able to find any spec sheet but I could try harder.
These types of facuets are commercially made and common in walmart restrooms and other similar facilitys (same for self flushing urinals and toilets). I noticed recently that a hand soap maker has an auto soap despencer that squirts out a little soap when the hand is passed under it. Probably less than $10.
Perhaps some consumer grade hardware somewhere, I was just doing a quick look around to see how the faucets were triggered and most of the ones I found were in the 800-2k USD range, it was a lot more than I expected.
You're right, the consumer products are expensive. I want to make one with IR sensor so if I stick my hand out to trigger the sensor, arduino turns on or off the water. I'll give it a try.
@liudr
That is a good found. I look at the site, and at the part picture, since it is design to use in washing machine, it probably use 120 V AC ( my guess ) one side is cold - , one side is hot and both ( hot/cold). Let say :
A - Cold B - Hot
1 0 = Cold water
0 1 = Hot water
1 1 = Both water ( my guess )
Need relay or TRIACS to control the AC. You may need to experiment/test. I hope you are confortable installing 120. And 
Plumbing !!! I don't like doing plumbing. !! And work in tight place !! I hope you are also comfortable working with plumbing.