Apple doesn't like apps repurposing buttons, so Camera Mic went ahead and did something creative: it turned your microphone into a button. All you need to do is run your finger across your iPhone's mic or headset mic to snap a picture.
It works very well, but the obvious downside is triggering a photo when you don't mean to. Loud noises can set off the shutter, but that only occurs when those noises are in close proximity to the mic. While the default settings are right where you want them, you can adjust the mic sensitivity in the app if you're having trouble.
Camera Mic is available right now for $1.19 in the iTunes App Store. Thanks for the tip, Matthew!