Require password from current user on front-end form? I have a controller that handles some font-end form validation. From that controller I’d like to require the current (already logged in) user’s password to update any field. How can I do that?
You'd create a Craft plugin with a controller that has an action, say actionSaveForm().

Your controller would look for a password POST variable and if that existed, validate that against the currently logged in user's account to make sure it was a match.  If so, continue onto Craft's saveUser or saveEntry, etc. (whatever you're using the form for, otherwise, return a validation error message.

Roughly like this:

public function actionSaveForm()
{
    $this-&gt;requirePostRequest();
    craft()-&gt;userSession-&gt;requireLogin();

    $password = craft()-&gt;request-&gt;getRequiredPost('password');
    {# grab anything else you need from POST #}

    $currentUser = craft()-&gt;userSession-&gt;getUser();

    if (craft()-&gt;users-&gt;validatePassword($currentUser-&gt;password, $password))
    {
        {# saveEntry, saveUser, etc. then redirect #}
    }

    // Invalid Password
    craft()-&gt;urlManager-&gt;setRouteVariables(array(
        'error' =&gt; "Invalid Password"
    ));
}