login block without a "log in" button
The jquery approach to achieve this one which worked for me (drupal 7) is the following code

$("#edit-pass").keypress(function(event) {
  if ( event.which == 13 ) {
    $('#edit-submit').click();
   }

});

$("#edit-name").keypress(function(event) {
  if ( event.which == 13 ) {
    $('#edit-submit').click();
}});


#edit-name and #edit-pass are my two login fields.