When prospects fill out forms with information, where does that information get stored? There doesn't seem to be a report that does this, and you can't make a custom report without knowing what criteria to pull.
That information creates a new lead in Marketo (or if it was an existing lead updates that lead record). If you were to create a Smart List with the Filter of Fills Out Form, you could get a list of all people who filled out a form. Is that what you're looking for?