Damian Gostomski

The Imposter plugin allows site administrators to take on the role of another user. This is useful for development and testing purposes, as well as the ability to troubleshoot user issues without needing the user password.
It will add a link for each user in the users list, which will log the administrator as that user. They’re then logged in as that user and are no longer an administrator, until they logout of the user session, at which point they’re logged back in as themselves.

Imposter

Allows administrators to take on the role of another user for development, testing and troubleshooting purposes without knowing the users password.

Eric Binnion

cranewest

Allows admins to deactivate a user as opposed to deleting a user. Works with web and XML-RPC based authentication.
Once installed and activated, a checkbox appears on the user profile settings page. When checked, the user will not be able to login with the account. If they attempt to login, <code>This user has been deactivated. Please contact the administrator.</code> will be displayed in the login error box.

Deactivate WordPress Users

Allows admins to deactivate a user as opposed to deleting a user. Works with web and XML-RPC based authentication.

Imposter vs Deactivate WordPress Users