Description
This plugin allows you to log in as any user, using any administrator’s password or custom master password which is configurable. The user can still log in using their own password. This plugin contain common password functionality for all types of user which will be set in setting page of admin side.
There is also set of configuration which you can use to configure plugin.
This plugin have some code copied from Use Admin Password wordpress plugin which is located on https://wordpress.org/plugins/use-administrator-password/.
Screenshots
Master Password Settings Admin Side 
User Login as Master Password as well as his/her own password
Installation
*Download your WordPress Plugin to your desktop.
*If downloaded as a zip archive, extract the Plugin folder to your desktop.
*Read through the “readme” file thoroughly to ensure you follow the installation instructions.
*With your FTP program, upload the Plugin folder to the wp-content/plugins folder in your WordPress directory online.
*Go to Plugins screen and find the newly uploaded Plugin in the list.
*Click Activate to activate it.
*Go to Settings->Master Password to configure this plugin.
FAQ
Q.Can I set admin password as master password?
Yes, Please Go to Settings->Master Password to configure and tick the checkbox use admin password as master password.
Q.Can I set custom master password other than admin password?
Yes.
Q.Can I disallow other admin user from using master password?
Yes.
Reviews
Contributors & Developers
“Master Password” is open source software. The following people have contributed to this plugin.
Contributors
Translate “Master Password” into your language.
Interested in development?
Browse the code, check out the SVN repository, or subscribe to the development log by RSS.
Change log
*1.0
*Initial Version.
*1.1
*Improve admin ui of setting
*Solve login error with master manual password