tethering - How can I control a DSLR camera programmatically?

Is it possible to get the code (in any programming language) to give control over the features of a DSLR e.g shutter speed and exposure?

Canon and Nikon cameras come with a pre-included software that can allow to control the camera from the computer but I want the specific commands that I can send to the camera for it to vary its function and take pictures and send back to the computer.

I'm particularly interested in the Nikon D300, Sony Alpha 700, Olympus E300, Canon EOS 20D, 70D and Pentax K10D.