Drupal 7's new and improved Form API makes using AJAX a breeze. It enables you to add, replace, or remove form elements via AJAX without ever really having to get your hands dirty.
I recently had the opportunity to give it a whirl. Here's a the deal:
I have content type
Resume and Profile2 type
Job Seeker Profile. I'd like my users to be able to create up to 5 resumes, and I'd like them to have the option of importing information from their profile into their brand-spanking new resume. So, I'm going to create an 'Import from profile' button that will populate the resume fields with profile field values via AJAX.
Note: It is possible to accomplish something like this by setting default values via a token. However, in this case, I'd like to be able to import things like field collections, addresses, and fields with unlimited cardinality. Here's the code that accomplished it!