.NET is not a language. You always have to pay, but sometimes you pay directly and up-front.
Re:Introducing Bing Code Search for C#
by curiousDog on Monday February 17, 2014 @03:32PM
(#7254312)
.NET is a framework. This is how the stack looks like: http://en.wikipedia.org/wiki/File:DotNet.svg.
Sorry, I updated my comment.
I also should have mentioned Mono, but that's a different topic.
I don't think it's worth mentioning Mono. It's impressive what they've managed to accomplish, but it's nowhere near being a true replacement for .NET, especially for any serious system.
Microsoft makes most of its money with its business products, so it's not cheap. Some companies just simply don't want to pay the Microsoft premium.