PeoplePress

It is currently in early stage of development and provides very basic functionality. 
We are looking forward to add more functionality with our upcoming releases.
Currently it offers login, password reset and a basic user profile with default fields.

It is lightweight user profile and account management plugin.

jrking4

This plugin adds an option to the User List in the admin area where you can click “Masquerade as User”, you will be automatically logged in as that user and redirected to the home page.

Masquerade

Adds a link to users.php that allows an administrator to login as that user without knowing the password.

PeoplePress vs Masquerade