The iPhone. It wasn't terrible to begin with (a fancy phone with a decent web browser), but it became a whole different product after the SDK was released.
What turned the iPhone into a serious product, from my own point of view, was the regular stream of updates and bug fixes that Apple provided. I didn't like my iPhone very much at first, but it just kept getting better, through no effort or expense on my part. I wasn't accustomed to buying electronic devices that magically got better over time. They won at least one loyal customer with that approach, and influenced the way I do business today.