Tk is a graphical user interface toolkit that takes developing desktop applications to a higher level than conventional approaches. Tk is the standard GUI not only for Tcl, but for many other dynamic languages, and can produce rich, native applications that run unchanged across Windows, Mac OS X, Linux and more.
See tutorial and example.
Ranked in these QuestionsQuestion Ranking
Pro Great for beginners
Easy to pick up and run with it.
Pro Easy and good looking in Mac, Linux and Windows
Pro Bindings for several languages
Bindings for Python, C, C++, Ruby, Perl, Go, Java, Haskell and more, see Languages with a Tk binding.
Pro Easy styling
Once you understand how to style, the styling can look great.
Con Base looks are garbage without a good amount of styling
Going with the base look for your app is a bad idea because it looks like garbage with the defaults.
Con C# support lacking
Although the Eagle Project exists, which offers Mono and .NET Core support, C# support is not standard.