Hugin is a good open source tool for creating panoramas of your overlapping photos of a scenary.
Example: combining 3 sky photos
Hugin contains several options for combining the images. The figure below is obtained after making manual alignment of the selected points (stars) in the photos.
The sky photos are, from left to right, shown below:
The jpg version of the result is shown here (click to enlarge).
The orignal result was big 26MB tif file. No other editing nor filtering is done on the photos, neither there were no filters used in the digital pocket camera. (Except resizing to 50% due to blog upload problems).
Stellarium was used to identify the constellations on this photo. The result is shown below (click to enlarge).