Hi Phil
I use
Neat Image (as a Photoshop Plug-in), although I have only used it for happy snap daytime photography, not astro photography. It does what it says, reduces noise and it seems to work quite well. I have only used it on a casual basis so I cannot really provide any in-depth advice on its effectiveness compared to other software apps.
You can profile your camera’s noise and save it, for the various ISO speeds as well.
I suspect that skillful use of the Photoshop’s own noise reduction features would probably do as good a job for most applications?
Other apps I am aware of are:
Noiseware
NoiseNinja
Cheers
Dennis