NetGore is a free, open source online RPG engine created with C# and SFML. It is designed to provide a strong and flexible foundation for your own 2d online games. While the focus is towards side-scrolling and top-down RPGs, it can still be used to create any kind of 2d game that does not require very low latency (shooters) and uses the client-server model.
This is a fork, you can find the original repository here.