How do I find users by password?

The user password (stored as an MD5 hash) can be retrieved like so:

$users = get_users();

foreach ( $users as $user ) {
    $password = $user->user_pass;
}

Assuming you have some known value for passkey, you can hash it and compare it to each user’s password:

$passkey = 'somestring';

$hashed_passkey = md5( $passkey );

$users = get_users();

foreach ( $users as $user ) {
    if ( $user->user_pass == $hashed_passkey ) {
        // We have a match!
        // Do something
    }
}