You can easily write good Windows applications on Linux and OS X; and Microsoft distributes standard virtualization images for free which you can fire up in a V

You can easily write good Windows applications on Linux and OS X; and Microsoft distributes standard virtualization images for free which you can fire up in a VM to test your Windows applications. In addition to those virtual machines, Wine implements much of the Win32 library and can be used to test your Windows applications with some minor limitations. For iOS you cannot run your build process on a remote machine without jumping high hoops, you cannot build without OS X at all, you cannot sign code for your own device without their IDE and a certificate from their website.