GPhoto2
libgphoto2 is a portable library which gives access to hundreds of digital cameras. It can be used in any software that wants to control digital cameras settings or simply take some pictures with them. A list of projects using it is available here.
There is a command line interface to libgphoto2, named gphoto2. It can be used to verify what settings are available through the USB link of your camera but also to take pictures or make a simple intervalometer in shell programming. Pictures can be taken in the internal memory of the camera or transferred right away on the computer.
Coming soon: list of main commands and settings for astronomy.