SuperTux is a free classic 2D jump'n run sidescroller game in a style similar to the original Super Mario games.

SuperTux Features:

a nearly completely rewritten game engine based on OpenGL, OpenAL, SDL2, ...
support for translations
in-game manager for downloadable add-ons and translations
Bonus Island III, a for now unfinished Forest Island and the development levels in Incubator Island
a final boss in Icy Island
new and improved soundtracks and sound effects
new badguys, bonuses and power-ups (air-, earth- and ice-flower)
a halloween tilemap
new graphic effects (glowing objects, particles, ...)
levels and worldmaps are scriptable using squirrel
much more game objects: trampolines, switches, portable stones, wind, moving platforms, ... - most of them have scripting APIs
improved statistics
many invisible changes, like unit tests, efficiency improvements and more bugfixes
much more...