It's a widescreen world, and there is no better way to earn the scorn of the Internet than posting a video shot with a smartphone in portrait mode. However, it's often easier to hold a device in portrait orientation when filming a video, especially if you only have one hand free. That's where Horizon comes in. This app uses your device's gyroscope to make sure the video is always in landscape, even when the phone isn't.
Horizon is super easy to use - simply open the app, and it instantly starts adjusting the frame to keep it level. No matter how you turn the phone or tablet, it stays is landscape. There are three different modes in Horizon to control exactly how the cropping works. The default is fixed size, where the frame is one screen width across no matter the angle. The rotate and scale mode adjusts the video frame to use the maximum resolution at any angle, but can result in a zooming effect if you rotate during filming. Then you can disable the rotation completely.
Horizon allows you to choose video resolutions up to 1080p, but it's probably rendering up to that resolution after the capture. The cropped frame is a lower resolution than if you were filming in landscape at full quality. With rear cameras, the difference shouldn't matter too much, but a front-facing sensor is already working with a much lower resolution.
In addition to recording standard video, there are a number of cool filters that you can pull up in a live preview grid. The built-in gallery supports AirPlay mirroring as well. It can even run AirPlay while recording. Horizon works with the iPhone and iPad.