Rewind Staging , 95 reviews
Pretty easy to copy a site so you can work on it without messing up your live site, and you can then set it up to push the changes back (with some caveats that are more about Shopify API restrictions mainly affecting products, but nothing that cannot be tweaked afterwards). The support is responsive when I was having some issues and I had some questions as well.
I do agree with others on here that $99/mo is a bit much, but if you can do what you need within that time frame its actually not so bad.
Replay has made the seemingly impossible task of maintaining and syncing multiple e-commerce stores (product info, inventory etc) possible.
I used the app to copy one store and keep it updated on 2 destination stores and it works great. Even the themes were copied perfectly. The only reason I don't give 5 stars it's because you have to subscribe in the source store making it a very expensive App to keep.