Free Video Rotator is a simple tool for rotating videos by 90 degree increments, without quality loss.

It's extremely easy to operate. Open a video, tap "Clockwise Rotation" to set the amount to rotate (90, 180, 270 degrees) and select Start to output the rotated footage.

While this worked for us, one notable disadvantage is that when you open the video, the program displays the first frame only. If this isn't representative of the full clip - it's all black, say - you won't see any change when you hit the "Clockwise Rotation" button, and won't be able to see exactly how the video will look until you've created it.