Piskel is an excellent tool for it, just make sure your browser is compatible with it, I've encountered problems before where saving duplicated colors in the sprite's palette that I had to spend a day or two to fix. This can really screw you over when you have to make MP skin colors for your character work.
Firefox is the safest bet for the choice of internet browser to use if you're planning to use Piskel as the browser-based alternative over programs like GIMP.