This is an implementation of the inverse compositional image registration algorithm for Gimp. Image registration, also known as image alignment, is the process of transforming a given image to achieve a best match with another image. The need for such a tool is given in situations where you want to combine two or more images which are similar, but geometrically do not have a perfect match.
A typical use is the combination of 2 or 3 photos in HDR methods. Unless you use a stable tripod, the source images will always be slightly out of alignment, so the very first step will be to re-align the images. The alignment step is normally built into dedicated HDR software, but for doing simple HDR (using for example Gimp's layer masks), you'll need to align the source images yourself, and this is where this plug-in can be helpful.
The plug-in is hosted at http://sourceforge.net/projects/gimp-image-reg/, where you can download the binaries for Win32 and Mac OS X (10.5 and 10.6). For building the plug-in for Linux please download the sources and follow the instructions in README.