Conga Composer is a document generator that lets you create templates & produce consistent, on-brand documents automatically in one click, using any standard or custom Salesforce object. It is available on the AppExchange.
You can make a one-click solution or automate via workflow/on a schedule. Merge data from external sources, like an ERP or multiple Salesforce instances. The Composer API offers total flexibility.
Listing details can be found here.