When you take photos in low light, your pictures end up with some noise. The amount of it depends on how dark it was when you took your picture and how you adjusted settings on your camera.
A lot of noise will be very difficult to remove, however if your photo doesn't carry too much of it, there are ways to clean it up.
Denoise not only detects areas with noise on your pictures but it lets you correct them, thanks to a panel of seven different sliders, including noise scale, geometry regularity and spatial step.
Denoise doesn't offer any quick link to iPhoto, however you can drag and drop pictures onto it. Noisy areas are shown by little bees on your picture.
Changes come into effect as soon as you apply them. Unfortunately you won't be able to zoom in onto pictures to see them in more detail.
Despite lacking quick access to iPhoto or a proper zoom, Denoise is good at identifying and removing noise from your pictures.