Rewind Copy (formerly Replay)
Store Copy Made Simple. Duplicate and Manage Multiple Stores.
Popular with businesses in the United States
Seamless workflow: use directly in Shopify admin
Speed tested: no impact to your online store
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.
Replay has made the seemingly impossible task of maintaining and syncing multiple e-commerce stores (product info, inventory etc) possible.