The Panic Button app by Amnesty International was designed with journalists and human rights activists in mind, so that if they happen to find themselves in a threatening situation, a help notification will be sent to a pre-determined set of contacts.
If you're in need of aid, sometimes extreme discretion is important, which is why the app is disguised as a calculator. This way, if someone searches your phone, they'll be none the wiser. To call for help, users can either press the phone's power button rapidly five times or repeatedly press a number on the calculator.
The app was conceptualized during an open IDEO challenge in 2012 and it has since won the FreedomHack competition in 2013. Now, it's in beta mode and completely open-source, available for Android users to download through Google Play.