Both of the big powerhouses in the digital SLR camera market -- Canon and Nikon -- make software with which you can control your camera from your PC through an attached USB cable. It is a nice tool for situations when manually triggering the shutter might cause shake (such as long exposures), or for automating multiple shots at timed intervals. But, naturally, the camera makers don't supply this software for free operating systems -- and just as naturally, the open source world has an answer. You can perform the same remote operation functions using gPhoto.
The gPhoto project is one of those open source gems that works so well that it is easy to forget it is there. Plug in a digital camera, and the photo management app of your choice -- be it F-Spot, digikam, gtkam, or another -- identifies the camera, then quietly and expertly retrieves your photos with little to no fuss. But for most cameras, gPhoto can do considerably more, including just about any function supported by the camera's firmware. That includes USB image capture, a function normally thought of in the context of always-attached webcams, but available for nicer digital cameras, too.