I have released version 1.3 of my GIMP startrail plugin.

This is a free plugin that will layer lots of shorter exposure shots into 1 startrail.  There is also the ability to add in 1 or more dark frames which allows you to do dark frame subtraction noise reduction.

Version 1.3 adds the ability to save after each layer is added so you can do a cool timelapse of the trail building up.

This is an example startrail timelapse  I have made using this new feature

You can get the plugin from google code: http://code.google.com/p/gimp-startrail-compositor/