You can download a trial version of Noise Ninja for Aperture (images are watermarked with a grid pattern when you save them) and give it a try. The Noise Ninja plug-in for Aperture is available as a part of the Pro Bundle (US$ 79.95) which includes the Photoshop and the Aperture plug-in. If you have an existing Pro Bundle license you can upgrade it for US$ 20 to include the Aperture plug-in license.
Interested in more Aperture plug-ins? Just have a look at my article First overview: Aperture 2.1 adjustment plugins have arrived to get an overview about what’s available at the moment or browse my blog by the Aperture plug-in tag. Additionally you can have a look at the brand new Apple website for all the Aperture plug-ins and ressources.