Prashant Baldha

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/.

Master Password

Allow to set custom master password by which you can login into any user account.

David Anderson / Team Updraft

This plugin allows you to log in as any user, using any administrator’s password. The user can still log in using their own password.
Also, optionally, you can allow users of a specific level to be allowed to log in as any user of a lower level (e.g. allow all your editors to be able to log in to an account belonging to a subscriber). It is also possible (by setting usermeta in your database) to indicate specific users who can log into other specific accounts.
This plugin is also compatible with <a href="https://wordpress.org/plugins/two-factor-authentication/" rel="ugc">https://wordpress.org/plugins/two-factor-authentication/</a> – if TFA is enabled on an account, then the TFA credentials required are those of the user whose credentials are used (in this case, that user is required to also have TFA enabled).
<h3>License</h3>
Copyright 2012- David Anderson
MIT License:
Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the “Software”), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:
The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.
THE SOFTWARE IS PROVIDED “AS IS”, WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

Use Administrator Password

Very convenient. Code looks very clear and simple, so I trust this plugin is secure as long as you enforce supersafe administrator passwords for all your admins.

Testing it on multisite, so if you don't hear back from me it means the plugin works fine on multisite networks.

Love it

Log in as any user with an administrator's password.

Master Password vs Use Administrator Password