iOS (jailbroken): Waiting for the camera app to load doesn't help when you're trying to take covert photos. QuickShoot is a jailbreak tweak that takes a picture from your home screen without opening the camera app.
After you install the tweak, you can take an image just by finding the Camera icon on your home screen and double-tapping it. You'll see a small badge on the app to show it's working. When it's done, you'll have that photo in your gallery.
Of course, this means you can't frame your shot, but it's good for snapping a secret picture without others knowing (something we've talked about before). You can change flash settings, choose which camera to use and turn on HDR in the app's Settings pane as well.