The wavelet/JPEG 2000 plug-in, written by Divyanshu Vats, from the 2006 GIMP Google Summer of Code wavelet project, mentored by Simon Budig.
It creates three plug-ins:
jp2: JPEG 2000 support
denoise: A noise removal plug-in
ihalf: Inverse halftoning -- remove halftones from printed images.
It requires the openjpeg library, available at: http://www.openjpeg.org/
I'm not sure what the license is (the code files don't say) but the COPYING file looks pretty free.