As one of the flagship Android phones, the Galaxy S4 includes a bundle of different Samsung features. Not all of them were fantastic, as Tim Schiesser observes in his review, but the ability to take a call without touching the screen is a novel idea.
'Air Call Accept' on the Play Store offers the same functionality to any Android device with a proximity sensor, going as far back as Android 1.6. The app has some pedigree, having been developed by the same people as WiFi Mouse. The paid version of Air Call Accept is £1.95, and it includes the ability to send email with rejected calls, reject calls, and enable speakerphone upon pick-up.
As of right now, reviews give the free version an overall of 4.0, with users confirming the app works on a handful of phones. Among those phones are the S3, Note 2, and Oppo Find 5. Not every phone has been a success however; one person claims it does not work on their Droid Razr Maxx.
If nothing else, it's nice to have the functionality made available outside of Samsung's flagship.