GrafX2 is a good application for PixelArt, even though its user interface is somewhat unconventional. It is free (FOSS) and runs on both Linux and Windows (and possibly also Mac OS).
Here are a few URLs related to GrafX2 (links are alive at the date of this message’s posting)
Linux App’s Installer: Flathub—An app store and build service for Linux
Description: http://grafx2.chez.com/
Tutorial (video): GrafX2 An Introduction - YouTube