If you have a Canon camera you should--if you haven't already--check out the CHDK (Canon Hackers Development Kit). The CHDK is a software package that is installed on an SD memory card and runs on your camera in conjunction with your existing firmware to allow a whole slew of features heretofore unavailable with the stock camera. In other words, your camera can do a lot more than Canon is willing to tell you or support with their software, e.g. greater control of photos and video settings, ability to record RAW files, adding USB remote shutter trigger(the subject of this Instructable), and more.
It does all this without voiding any warranties. There are no firmware or hardware hacks. It's all done with software and can be turned on and off in a matter of seconds. It's free, open source and allows the ability to use existing as well as write your own scripts to give even more functionality. Best of all, it gave my Canon A590 IS an option for remote shutter control. Alas, this only works for Canon cameras. If you use a Kodak, you're out of luck on this particular site.
Note: This Instructable is in conjunction with two others recently posted by the Author. They are pieces of this project made into their own Instructables. They combine like Voltron to form the end result of this one. There may be a bit of overlap in these Instructables. Just skip what you have already read. Links to these other Instructables are found on the relevant steps.