Imposter

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.