A very tiny plugin that does only one thing: It adds a field to Gravity Forms that will allow you to collect the last four digits of an American Social Security number more easily. While you could use a regular text or number field for this, this field makes it more clear that you are, in fact, asking for the last four digits. It will also do some basic validation for you to ensure that the input is indeed a number.
Beyond this, not a whole lot more to say about it. And I hope this is needless to say, but please do be careful with the personal data you collect.