You might be thinking that how come Microsoft overnight decided to go Open Source. But that's not true, Open Source is not new for .Net. It has started all the way from April,2014 when they have formed .Net Foundation (http://www.dotnetfoundation.org/), where they released a number of .Net components as open source.
As you can see below there are a number of projects which are open source including Entity Framework 6, NuGet, .Net Micro Framework etc...
It will take some time before all of the .Net code goes Open Source but not more than few months as you can see above many main components are already made as open Source.
Asp.Net 5 will be available for Mac and Linus as well. It will include a web server for both the OS, same way as node works.
You can check more at http://www.hanselman.com/blog/AnnouncingNET2015NETAsOpenSourceNETOnMacAndLinuxAndVisualStudioCommunity.aspx.
Please Like and Share the Blog, if you find it interesting and helpful.