Android (4.0+): Some apps work better in portrait mode, and others work better in landscape. Swivel lets you set a permanent orientation for any app on your Android phone.
YouTube, for instance, is almost always better in landscape mode. By default, Android only lets you set your phone to auto-rotate or not. You can customise your phone's orientation or even get some, admittedly complex, control over individual apps. But those aren't good solutions.
Instead, Swivel lets you set a preferred orientation for each app. Not just portrait and landscape, you can even specify the direction. In fact, you could even set your phone to not auto-rotate, but still set some apps to auto-rotate and override your settings. Neat!