The Terrain4 2d game engine is a c# game engine built on the windows forms .net framework, it was made to make game development with windows forms much easier, and is also aimed to help people learn c# with ease, it is still in active development so it is not currently available to download quite yet as it is early in development, but will be completely free to use.
An example of its planned features are: a fully featured terrain level editor with layers and parallax backgrounds, it can be tile based or image based as it separates collision from tiles, it can handle polygonal collisions(triangles) and box collision(squares and rectangles), and in the future, spherical collisions, it has noise classes built in to make terrain generation easier, it has frame based gravity, and a complex collision method so that it completely avoids all clipping, it also has been optimised so games made with it run atleast 60fps with full tile based levels loaded