The High Pass Filter plugin is licensed under the GPL.
Latest version is 1.2 (24 March 2008), which fixes some compile problems with recent versions of Gimp and/or gcc.
The source code is available here.
gimptool-2.0 --install hpf.c will compile the plugin and install it in your local Gimp plugins directory.
The plugin appears in the Filters/Enhance menu as High Pass Filter....
The high pass filter plugin is designed to smooth out the average brightness of an image, which can be useful when trying to tile a texture image. The sliders allow you to adjust the radius over which the average colour is calculated, and the (new) average brightness of the image. Let me know (at the email address in the code) if you have any problems or suggestions.