Log in a WordPress user programmatically

Here is the function, drop it in your functions.php file:

function auto_login( $user ) {     
$username = $user;    
if ( !is_user_logged_in() ) {         
$user = get_userdatabylogin( $username );         
$user_id = $user->ID;         
wp_set_current_user( $user_id, $user_login );         
wp_set_auth_cookie( $user_id );         
do_action( 'wp_login', $  user_login );     }  }

Then, to log in a user, do the following:

auto_login( 'admin' )

Thanks evilsocket for the code!