I use the Topaz Labs De-Noise Photoshop plugin for reducing noise in high ISO nightscape images.
This image shows the dramatic difference. Left side is straight out of the camera, untouched, from a ISO6400 nightscape image with the Canon 6D. The right side shows the same field after using the Topaz De-Noise plugin on the RAW Medium setting.
It's always the first step in my processing workflow.
Higher-res here:
http://bit.ly/W48RSx
It's a 1:1 crop.
Also attached is the original image straight out of the camera, no processing except noise reduction. Shows how much light is captured at ISO6400!
Original image: ISO6400, f/2.8, 30s exposure