Try Kate Gregory's Pluralsight courses.

I think the best book for complete beginners is "Jumping into C++" by Alex Allain. From there, move on to "C++ Primer Plus" by Stephen Prata.

Quantitative finance, scientific computing.

Recommended resources for learning C++ nowadays: "Jumping into C++" by Alex Allain, "C++ Primer Plus" by Stephen Prata, and Kate Gregory's C++ courses on Pluralsight. You can get a free 3 or 6 month Pluralsight subscription by signing up for Microsoft's free Dev Essentials program.

It might not be common knowledge yet, but C#, F#, and .NET are fully open source and cross-platform. Development is done in public on GitHub (https://github.com/dotnet), and pull requests are accepted from the community. You can even get a support contract from Microsoft and call them up to troubleshoot your .NET project on Linux. No Windows required.

One option with C# Windows Store apps is that they can generate native code using something called ".NET Native". https://msdn.microsoft.com/en-us/vstudio/dotnetnative.aspx

VS2012 for Windows Phone development is free: http://aka.ms/winphone8sdk

You still have to pay for it if you want extension support.

You may be surprised to hear that there's lots of stuff happening with open source here at Microsoft. ASP.NET MVC, the Azure SDKs, and F# are all open source. Check out this blog for more: http://blogs.technet.com/b/port25/.