Office 365
How to build a great Employee Directory in Office 365 & SharePoint
https://hubs.ly/H09P9ZK0
https://www.hyperfish.com/employee-directory
Setup PDF | Download the zip | Download the guide
References
http://sympmarc.com/series/create-a-simple-sharepoint-2013-employee-directory-on-office365/
https://www.puzzlepart.com/three-things-that-will-make-your-people-search-rock/
http://www.techmikael.com/2013/10/make-sure-your-people-search-is.html
https://en.share-gate.com/blog/corporate-directory-sharepoint-search
https://thomasdaly.net/2016/01/16/hiding-people-from-people-search/
http://www.techmikael.com/2014/12/how-to-trigger-re-indexing-of-user.html
https://www.eliostruyf.com/starter-javascript-display-templates-for-your-projects
SharePoint
Codeplex
Employee Directory Webpart for SharePoint 2007/2010/2013
http://employeedirectory2010.codeplex.com/
Source code or WSP for SharePoint 2007
Source code or WSP for SharePoint 2010
Source code or WSP for SharePoint 2013
Third Party
SharePoint Staff Directory Web Part
http://vinewave.com/Products/StaffDirectory.aspx
Articles
Simple Employee Directory using Search - People Directory
http://www.sharepointconfig.com/2013/05/how-to-create-a-simple-sharepoint-2013-people-directory/
Create a Simple SharePoint 2013 Employee Directory on Office365
http://sympmarc.com/series/create-a-simple-sharepoint-2013-employee-directory-on-office365/