Inkscape comes with a lot of extensions built in, and they are divided into categories. As with much of the Inkscape user interface, the extensions work in a way that is inconsistent with other areas of the program by opening in an independent dialog box. This is a shame because it would be nice to be able to quickly switch between extensions when experimenting with various effects. Extra extensions can be installed using the extension manager, and there are some additional ones on the website.
Krita has a system for Python plugins, but it’s not extensively populated and there don’t seem to be many plugins online. It also features a front-end to the G’MIC graphics processing plugin system, but these are focussed on the bitmap image side of the program.