Supertux is another project that has been around since I started getting into the FOSS-world. It has had a few releases now and then, and the latest one 0.4.0, is big! The whole engine has been rewritten to use modern tech, and content-wise there are new effects, levels, badguys, power-ups, etc, etc. There is also a built-in manager for downloadable add-ons and translations (support for various languages is another addition). The changelog is really long and can be read here. A real classic you should at least play once!
Classic 2D platforming at it's finest!
I love how these overworld maps add to a sense of exploration and progress.