Asesprite is open source, you can compile the source code yourself and then it is free.
Viewing post in Mystic Woods - 16x16 Pixel Art Asset Pack comments
Not sure if you're still looking but you could try something like pixilart or piskel. I believe they're free and on the web. There's also libresprite which is basically a free aseprite.
I've compiled aseprite from source myself and honestly it's worth just paying if you can spare the money. it's a great pixel art program.
I haven't used krita myself but I hear it's in general a good drawing program. Might be worth a look.