@mauve Definitely Aseprite! It's the cutest! Unfortunately it's not open source anymore, but the source code is still public, and you can find free binaries for all mayor operating systems on the Internet. Try the pixelart subreddit.
@mauve Ah, for Linux: Arch/Gentoo have source packages, for Debian-based distris try to find the newest deb you can.
@blinry I've been recommended Aseprite, but I didn't know it was (at one point?) open source, I'll definitely look at that as I am an open source nerd and think wrangling with build systems is a bizarre sort of fun.