User Switching plugin by allows you to quickly swap between user accounts in WordPress, WordPress Multisite, and WordPress MU, at the click of a button. You’ll be instantly logged out and logged in as your desired user.
Instant switching and redirection to the WordPress Dashboard.
Instant switching back to the originating account.
It’s completely secure (see the “Security” section below).
Compatible with WordPress, WordPress Multisite, and WordPress MU.
Only administrators can switch to another user. Lower level users cannot switch between accounts.
User switching is protected with the WordPress nonce security system, meaning only those who are allowed to switch users can switch.
Full support for administration over SSL (if applicable).
Passwords are not (and cannot be) revealed.
Download and info: http://wordpress.org/extend/plugins/user-switching/