The 3 most successful hardware add ons are Kinect, Wii Balance Board and the N64's RAM expansion pack.
Kinect because it has the most number of titles that support the add on: ~130 upcoming and existing games.
The Wii Balance Board because of the number of units sold: ~34 million units sold.
The RAM expansion pack because of the percentage of total system games that supported it: 15.5%...60 games out of 387 titles.
Significant support is subjective though.
The rEVOLution is not being televised