iOS (Jailbroken): If you've ever wanted to secretly take a picture of something with your iPhone without anybody knowing, the default camera app is not the way to do it. But if you've jailbroken your phone, you can install SlyCam, from Cydia, to take photos from your Notification centre.Once installed and activated, you'll get a new + icon in your Notification centre. Hit that, and a photo will be taken and saved to your Photo Stream, without any further indication that anything has happened.
If you do want a miniature viewfinder, you can hit the power button to the left of the + button and one will pop up. Bear in mind that having a visible viewfinder somewhat defeats the purpose of secretly taking photos with your phone.