One we don't cover or hear about too often is SuperTux, a free and open source platformer that's been around for some time now and it's getting a Steam release.
"Run and jump through SuperTux, the sidescrolling 2D platformer starring Tux, the Linux mascot. Squish and knock out enemies, collect powerups, and solve platforming puzzles throughout the Icy Island and the Rooted Forest, as Tux tries to save his beloved Penny from her kidnapper, Nolok!"
- Platforming gameplay similar to original Super Mario games, with some unique abilities such as backflipping and dynamic swimming
- Lovingly hand-crafted graphics contributed by a variety of artists, alongside engaging and catchy music
- Engaging levels designed with casual gameplay, puzzling and speedrunning in mind
- Weird, quirky and some not-so-adorable enemies that might be too cute to kill
- Two full worlds packed with unique and challenging levels, castles, and boss fights
- Other contrib levels, including seasonal worlds, storyless bonus islands, and downloadable Add-ons, which feature new and unique stories and levels
- Simple, flexible Level Editor, which allows for the creation and sharing of levels of any complexity