Xenko (formerly known as Paradox 3D) is an open source C# game engine. It is growing quickly and is supporting multiple platforms, with more to come.
Public roadmap visible at https://trello.com/b/FwbjOjjB/xenko-roadmap
Ranked in these QuestionsQuestion Ranking
Pro Good looking and fast
Looking nearly as good as Unreal Engine 4, but rendering significantly faster.
Pro Easy to learn and use C#
Pro Straightforward editor
Paradox provides a simple and clean working environment for designing games.
Pro No royalties or licensing fees
Pro Supports Visual Studio
Con Editor is Windows only
Since version 1.7, Linux can be targeted for runtime. The editor for the engine is available only to Windows though.