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.

EranSch

Allow WordPress administrators to masquerade as other users on their site. Adds a dropdown select box to the admin bar as well as individual links in the dashboard’s user view. WP Masquerade also allows the user to revert back to their previous session for easy return to the admin user.
Thanks to JR King (castle-creative.com) for the initial development of the plugin was forked from.

WP Masquerade

Allow WordPress administrators to masquerade as other users on their site. Adds a dropdown select box to the admin bar as well as individual links in  …

PeoplePress vs WP Masquerade