Chaos Tool Suite is one of the best API module to use. The module mainly provides API.
I've worked on ctools before to create multi-step form and show that in popup. This time I'm working on a project which has a Quiz in it which is built using ctools.
Drupal 7 field module defines a field level template file, which defines the HTML structure of the field. We can use this file to override the display of field including changing how label will be display.