One thing that was still missing from the Open Source release of .NET via .Net Core was an open source version of Microsoft's GUI projects WPF, Windows Forms and WinUI, which Microsoft has now done and announced on their Windows blog, together with a preview release of .NET Core 3.0 which now supports WPF, Windows Forms and WinUI.
On Hacker News, people were quick to point out that Microsoft doesn't accept patches that would make the projects work cross platform, so they will stay Windows only, and that the projects "Collect usage data and sends it to Microsoft".
So this isn't as awesome as it could be, but still really nice to see Microsoft releasing more and more components of .NET as Open Source.
More information can also be found on the .NET blog, including a tutorial on how to use .NET Core with WPF or Windows Forms: