Theming .NET WinForms (edit)
https://github.com/ComponentFactory/Krypton
https://github.com/IgnaceMaes/MaterialSkin
https://github.com/saneki-discontinued/FlatUI
I guess when you talk about "UIs done in WinForms" you mean UI or widgets framework?
Free:
OpenTheme: A free basic theme library.
Dock Panel Suite: Similar to the panel system in Visual Studio 2003.
RCM: Another free skin library.
XPTable: An extended list view.
And you can find a lot of custom controls in a place like CodeProject.
But, if you are willing to pay you can find gorgeous UI frameworks on the market. Generally, they offer a free trial:
...and many more.
