G’MIC, GREYC’s Magic for Image Computing is a full-featured open-source framework for digital image processing. It includes a wealth of image processing filters (about 600 at the time I write this), and is available as a stand alone command line package as well as plugins for GIMP, Krita, Photoshop, Affinity Photo, Paintshop pro, Paint.net. You can also use it online https://gmicol.greyc.fr/ though it can be slow depending on the load on the server at the time.
You can find out more at the GMIC website https://gmic.eu/. There is a gallery showing some of the effects at https://gmic.eu/gallery/ (hover over the thumbnails to see the original, move the mouse off to see the filtered version).