Easy Cow and Chicken Whistle is a fast and free app to generate sounds your cows and chickens can hear. Use this to train your farm animals…. maybe. You can also use it as a frequency generator. (FYI: Cows can hear ultrasonic sounds up to 30,000 hertz, however chickens can only hear in an auditory range up to 2000 hertz).
You can download it from the Android marketplace here.
Update 4/2012 This is now open source and hosted on GitHub. Check it out at my GitHub page.