I wanted to practice my stagnant drawing skills and something I always liked is pixel art, but I don't know any program for that (beside Paint and Photoshop) so I would like for recommendations here, the only big feature I need is multi layers management and that preferably runs in multiple platforms (You know, like Gimp and Inkscape).

Anything that runs on Linux/Windows/Android is welcomed. I guess there should be open source apps for that, and since it's only for practice I would to waste money on paid apps.