It's very easy to forget the orientation of your phone when recording videos leading to clips you have to watch with your head cocked to one side or holding your screen at 90 degrees. Like this example:
It is very easy to edit the video and rotate the image to its correct orientation and avoid the neck ache using Quicktime Player (I used version 10.1).
Step 1 - 'Right click' on the file and open with Quicktime Player (just in case it's not the default player)
Step 2 - Click on Edit -> Rotate Left or Rotate Right depending on the orientation of the original clip (in the example here I rotated right)
Step 3 - Export the file in one of the following formats
I used 480p in this example to match the input file and the resulting correctly orientated clip is below. The filesize shrank from 5.2MB to 3.6MB as a mov file. Both input and output files were H264 Mpeg4s the only difference I could find is that the audio sampling rate went from 32kHz to 16kHz.