iPhone (jailbroken): The fingerprint scanner on the iPhone 5s only offers a few functions. Thanks to a jailbreak tweak called BioProtect, you can now lock down specific apps as well.
BioProtect installs through Cydia like any other option — just search for it, pay $3 and download. It doesn't create an app, however, but rather exists in your iPhone's Settings app. From there you can choose a theme to match the colour of your phone (or not) and which apps to protect. Any app you enable will require a fingerprint scan each time you open it up. You can also toggle a few little things like vibration on failure and configure your Touch ID settings. Other than that, it doesn't do much else.
For $3, however, you get a lot more control over the security of your apps. Now we just need to get something that allows disabling the fingerprint protection where we don't want it.