You can check out the two stores here: Docs and Sheets.
To browse add-ons directly without navigating to the stores first, just hit “Get add-ons” in the Add-ons menu of any document or spreadsheet. Add-ons in the stores are created by developer partners that give users more features in their documents and spreadsheets.
when you install an add-on, it will become available across all of your documents or spreadsheets, and you can start using the add-on in question right away;
The Docs and Sheets add-ons are powered by Google Apps Script, the company’s server-side JavaScript platform with virtually no startup time.
However it should be noted that add-ons are in preview right now, but the tools and APIs are available to everyone. You can build a prototype for Docs or Sheets and then apply to publish it.
If you create a script that is distributed as an add-on, Google promises the following benefits:
- Better discovery: Apps Script has long been popular among programmers and other power users, but difficult for non-technical users to find and install. Add-ons let you distribute your code through a polished storefront—as well as direct links and even Google search results.
- Sharing: When two people collaborate on a document and one of them uses an add-on, it appears in the Add-ons menu for both to see. Similarly, once you get an add-on from the store, it appears in the menu in every document you create or open, although your collaborators will only see it in documents where you use it. For more info on this sharing model, see the guide to the add-on authorization lifecycle.
- Automatic updates: When you republish an add-on, the update pushes out automatically to all your users. There’s no more hounding people to switch to the latest version.
- Share functionality without sharing code: Unlike regular Apps Script projects, add-ons don’t expose your source code for all to see. That’s reassuring both to less-technical users and to the keepers of your codebase’s secrets.
- Enterprise features: If your company has its own Google Apps domain, you can publish add-ons restricted just to your employees. This private distribution channel is a great way for organizations that run on Google Apps to solve their own unique problems.
Credit: TNW