To browse through add-ons for Docs and Sheets, select Get add-ons in the Add-ons menu of any document or new Google Sheets.
Click "Add-ons" > "Get add-ons"
"Once you install an add-on it will become available across all of your documents or spreadsheets and you can start using it right away," according to Google Drive blog. You can always find them in the Add-ons menu.
There are add-ons that easily insert clip-art, find synonyms, create a table of contents, insert maps, create charts from spreadsheets, generate bibliography, check your writing for consistency, add mail merge to Google Docs, print address labels, remove duplicates, create random passwords and more.