.NET is now cross-platform.

http://news.microsoft.com/2014/11/12/microsoft-takes-net-open-source-and-cross-platform-adds-new-development-capabilities-with-visual-studio-2015-net-2015-and-visual-studio-online/

They throw around terms like open-source, but that doesn't usually mean what you want it to when MS says it, so who knows. There's also a free version of VS for small teams which is apparently feature-complete or nearly so.

>>1
I use C# at work, so I'm kind of glad of this. There aren't great high-level options on Linux right now, so this will take the place of Java (for me) if I should ever have need of that sort of language in my personal projects. I just hope they manage to keep the performance they have on Windows, where C# is only slightly worse than native code (actually, "raw" C# is equivalent to native code in speed, but the various frameworks they tack on make it much slower for a large range of programs in practice).