New Namek is the new home of the Namekian race. It was an almost duplicate of the old planet Namek, even to the green water and blue grass.
During the Cell Games Saga, Goku used Instant Transmission to travel to New Namek and find a replacement guardian for Earth. Dende was selected, and happily accompanied Goku back to Earth.
In Return of Cooler, Goku and some of the other Z Fighters battled Cooler and his metal clones on New Namek. Cooler had been attempting to use New Namek as fuel for the Big Gete Star.
Later in the series, Goku visited New Namek to ask for the use of the Dragon Balls
to combat foes like Majin Buu