SuperTux is a classic 2D jump'n run sidescroller game in a style similar to the original SuperMario games.
still in devopment, it a fun collection of 4+ worlds(you get more with add-ons) many badguys, levels, and a story where Tux travels to save Penny from Nolok who kidnapped her.
current version: 3.3(development)
website:https://supertux.lethargik.org/
wiki: https://supertux.lethargik.org/wiki/
bug tacker(with a good collection of levels): https://supertux.lethargik.org/bugs/my_view_page.php
Have fun!