Dynamically adding fieldsets to a form

Hi,

I’m looking for a form plug-in that will allow me to dynamically add groups of fields to a form.

The user needs to be able to describe multiple products, but I don’t know how many products they will be describing, so I want the user to be able to click and ‘add new product’ button to reveal a new set of fields.

Is there a plug-in that will let me do this easily, or would it be a custom job? If it’s custom can you give me any pointers?

Cheers