Rot'n'Stack

This program is similar to many astronomy programs in that it is meant to take several images and merge them.

This process is used mainly on dark images to remove the noise level. The difference between this program and more professional ones are the following: it's free, it works with jpg (and png) format so you can use it with images taken with digital cameras, and finally it's much simpler. With this program you can for instance set your camera on a tripod, take a bunch of 30s exposures of the night sky (more time will lead to star trails) and stack them to enhance the contrast.