Google have recently launched Add-on support in Google Docs and Spread Sheets. Just similar to chrome web add-ons these small utilities will enhance the functions of Google's online office suite that is Google Docs and Sheet.
The major difference between Google Scripts and Google Add-on is scripts are more users friendly while add-on works behind the scenes. Scripts can work with any file in Google drive while add-on works just as chrome-add-on, particularly on the currently open file.
Although add-ons work flawlessly with Google Docs, their source codes are hidden, unlike Google scripts. Most importantly Google Docs add-on can be severe as they are a more open source and cause issues such as unsecured sharing files with unknown users, viruses etc. Therefore, it is advised to be more cautious while running add-ons of unknown developers and note their add-on reviews carefully.
Google Scripts works well with new and old spreadsheets while recently launched Add-ons works only with the new version of sheets. This could be little annoying for a long time Google docs users.
Below are some of the best Add-ons for Google Docs and Spreadsheets
Workflows from Letter Feed - Automatic assistance for getting approvals from reviewers through mail.
HelloFax - Send and receive faxes online.
MailChimp - Send mails directly from Google Doc instead of usual Gmail.
PandaDoc - Share a document with your client and get legally binding e-signature in a short time.
ProWriting Aid - Make your writing more perfect with word-like checking utility.
Avery's Label Merge - Merge Google datasheet into labels and name badges.
Charts - Build Charts from data easily.
EasyBib - Easy creation of bibliography and citations online through Google Docs.
Thesaurus - Check grammar and consistency of your self-made articles. Make your writing more perfect.
Mapping Sheets - Add-on will prepare a Google map with your place details made on a spreadsheet.
Analytics Canvas - Pull your analytics reports into Google spreadsheets.
Translate - Self-advising easy online translation in you Doc.
TextHelp - A highlighter for your Google documents. Prepare your own notes in Documents.
Abbreviation Lists - Use correct abbreviations in your writing.
UberConference - Create conference call with all Doc viewers.
Track Changes - Keep an eye on editing your documents.
Doctopus - One of the most favourite among teachers for project assignments. Easy tool for managing student projects online at one place.