I wanted to take photos with my cheap point and shoot camera which doesn't support WiFi or anything like that. For this i used the CHDK (Canon Hack Development Kit) which supports a function where you send 5V to the USB Port and it takes a photo.
I connect to the Receiver using generic bluetooth software for my android phone. With two options 5second delay and immidiatelly photo shot.
Components
1×
Bluetooth Receiver
Bought on Ebay ~ 5€
1×
ATMega 328
Or just use an Arduino
1×
USB Mini plug
Hard to find, or use an old cable