There’s no question .NET is a popular option for developers, but if you wanted to run .NET applications your server had to run Windows. Microsoft is in the process of making its tools even more accessible, and open-sourcing the .NET server stack is one way the company is hoping to win over more developers to its platform. Gigaom has a good rundown of the history and significance of this, but the practical upshot is anyone who wants to write cross-platform .NET applications will now have an easier time of it, and open sourced platforms are the way of the future. Also, Visual Studio is going free (but still only works on Windows).