iOS: WiFi Camera lets you take pictures on one iPhone using another iPhone's camera. It's perfect for using one phone as a remote shutter for group pictures.
You can use your iPhone's headset to take pictures from afar, and even use your iPhone to control a DSLR camera. But, if you need to snap a group shot or otherwise remote control an iPhone camera, the best way to do it is with another iPhone.
Each phone needs a copy of WiFi Camera installed, and they need to be on the same Wi-Fi network or connected via Bluetooth. When you both start up WiFi Camera, you'll each see what the other camera sees. It'll be pretty blurry, but tap the shutter button on your phone and both devices will get a full-res photo from the camera on your friend's phone. It couldn't be simpler, and it's a super easy way to wirelessly snap those hard-to-get shots.
WiFi Camera is a free download for iOS.