The G'MIC plug-in for GIMP proposes a set of various filters to apply on your images, including artistic effects, image denoising and enhancement algorithms, 3D renderers, etc.. It is a quite large plug-in, integrating a lot of different effects.
A simple plug-in to do fourier transform on you image. The major advantage of this plugin is to be able to work with the transformed image inside GIMP. You can so draw or apply filters in fourier space, and get the modified image with an inverse FFT.