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.
If you're finding DigiKam or F-Spot, two of the many photo organization and editing tools for Linux, a bit limiting you may want to give Lightzone a try. The software isn't free, and curiously, isn't available for purchase either, but judging by the 20-day demo version currently available, it could end up a serious contender in the hybrid photo editing/managing market.
If you have a point-and-click digital camera made by Canon, you may be able to turn on all sorts of features usually reserved for more expensive SLRs. That includes live histograms, depth-of-field calculation, under and overexposure highlighting, and -- best of all -- shooting your pictures in RAW. The secret is CHDK, an enhanced, free software replacement firmware.