[sixhobbits]

This feature is built in by default to some screenshot tools. Funnily I use it most often not to make screenshots look better, but just to add some padding to beat the bad cropping algorithms on Twitter, Slack, Telegram etc that otherwise only show a small piece of your image in the thumbnail instead of the whole thing.

I've also found Browserframe.io useful (add a fake minimalist browser to your screenshot, and can also add a background, drop shadows etc)