iOS - App To Change Aspect Ratio of a Video
Videon

It's a great app for the iPhone. 

Not only does it have some great camera features; like stabilization, and manual exposure/WB, but you can also use it's post feature on any clip in your Photos folder. 

You can crop, change aspect ratio, framerate, adjust rotation, pretty much anything... along with a ton of filters; color correcting tools and so on. 

Very easy to use. And real time playback of your file as you correct it. 

When finished, you can output your file by exporting it to a variety of bitrates and sizes. 

A must have!