You should check out the Open Pixel Platformer. It's a tile-based pixel art platformer like you want, it's volunteer-based, there's already some great material to work with, so you can hone your skills in matching existing art, very important for working commercially. And there is plenty left to do, so you can choose those areas you're most confident with if you want to build your portfolio, or the areas you struggle with for practice.

If you want to do some unpaid work, I think the OPP's a very good option, since it helps a community project, and doesn't put you in a position where you're undercutting other artists.