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 one startrail. There is also the ability to add in one 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 time lapse of the trail building up.
This is an example startrail time lapse I have made using this new feature.
You can get the plugin from Github