Authentication - As User
Have you ever needed to access your site as one of your users? As User makes that task as simple as can be. Their username - your password.
It really is that simple. With the As User plugin, a user in any group authorized in the plugin configuration is able to log into the site as almost any other user by using their own password.
Of course, there is that caveat "almost any other user". The plugin protects against authorized users logging in as each other - for example, Admin-A cannot log in as Admin-B, because that would be a recipe for disaster. Additional configuration allows you to specify groups which are protected from login via this plugin (Super Users are always protected from login via this extension).
Forget the complicated alternatives with their crazy interfaces, you don't need them. You already know your own password, just type it.