Aspect ratios are an insanely powerful visual tool, and aspect ratio templates are a great option for spicing up your work quickly and easily.
In one of his best blog posts, prolific editor Vashi Nedomansky shared examples of most every aspect ratio ever used in the history of cinema (there are a lot of them), and shared a link to some free templates that can be overlaid on footage to change the aspect ratio.
However, those templates were all designed for HD frame sizes, and this being 2015, seemingly no one gives a shit about HD anymore. So Vashi did us all a favor and updated the templates for our high-resolution present and our even higher-resolution future. His new template pack (which you can download by clicking the giant image below) comes with 8 popular widescreen ratios just as black bars png. They range from classics like 1.85:1 and 2.39:1 all the way to the obscenely wide 4.00:1. Each template comes in resolutions ranging from 2K to 6K.
Applying these ratios to your footage is insanely easy. All you've got to do is import the template with your aspect ratio of choice into an NLE, drag it onto its own video track atop your edited sequence, and vertically reposition any shots that need recomposing to fit the new ratio. That's it. That's all.