Pandora, which helps with making panoramic images by loading all the images, positioning them approximately, and adding layer masks. I always regret that I don't have a Windows binary to offer people.
I've been thinking for a while that it would be easy to do a Pandora plug-in in script-fu, so that any GIMP user could run it. The only reason Pandora was written in C is that it provides a modified file selection dialog allowing you to choose the files you want in sequence. But it's not like the UI for that dialog is any great shakes (it's always been confusing, even to me, and I wrote it), and of course it uses the old gtk file selector, which has been orphaned for quite some time.
The way to work around this in script-fu is to let the user open all the images as layers in a single image using the normal Open and Open as Layers functions, then transform that image into a panorama by resizing it bigger and moving the layers to the right place. Easy! It's been on my list to do for a long time, but I didn't get motivated to write it until this morning when I wasn't doing anything else and another Windows user showed up on #gimp asking about panoramas.
[ 19:02 Mar 19, 2006 More gimp | permalink to this entry | ]