Vera is the true calc lover's OS. The main idea behind it is that it consists of a very basic kernel that allows for file storage and some hardware control, on top of which other programs or plugins can be built.
The Banana Kernel is an open-source pedagogical operating system intended to provide useful means for teaching basic OS concepts, with the possibility of interaction with external modules developed by the teacher or student.
Falcon is a new operating system which provides a fast kernel (combined with a fast filesystem) as well as an intuitive GUI. The GUI is completely new and the user can configure it as he likes to and as he needs to.