This list is sorted by software, since software/camera compatibility limits choices: SDM, CHDK, gPhoto2, non-software (pneumatic, mechanical, electrical, etc.).
Using SDM (Stereo Data Maker, select Canon cameras only):
- Powered USB hub with switch (Tom Horsely): Splice a switch (button) into the power cable of a powered USB hub, plug SDM cameras into powered USB hub, push the button to send a 5-volt pulse to the cameras that triggers them based on SDM code -- http://home.comcast.net/~tomhorsley/har ... %20Trigger
- something similar to above, also by Tom Horsley -- http://www.diybookscanner.org/forum/vie ... 9209#p9209
- 2 basters and a lever: Using air pressure between two basters, tilting a lever that pushes the camera's button, implemented in Monson's Sticks scanner -- http://www.diybookscanner.org/forum/vie ... ?f=1&t=422
- Air trigger (note that the punching bag is not the trigger, the black tube descends from the metal frame in left edge of the photo) http://www.diybookscanner.org/forum/vie ... ?f=1&t=864
- Stock remote: some cameras come with remotes, and some of those remote/camera arrangements can trigger two cameras at once.