Perhaps the first was I ♥ NY. It was created in 1977 and seems to keep growing in popularity. The NY tourism board does a great job promoting it, and no doubt makes a considerable amount in licensing the logo for use on many types of goods.
As a result, there are a myriad of spin-offs. Including those below which are registered (or pending) at the USPTO. [Click marks for USPTO records.] Some make creative designs with a heart. Some play directly off the image of the iconic I ♥ NY design.
For a fun related read, see: Why Are Heart-Shaped Logos So Popular (Slate)