Another alternative is RxMUI, which is basically an Arexx script which has a MUI interface through the use of RxMUI libraries.
If you're familiar with Arexx, you can get some pretty neat stuff running without too much effort.
Most of my stuff I just write in C and try to avoid GUIs where possible because they're just big and scary to program with my limited knowledge. :-)
But as mentioned, the MUI examples are a really good place to start. Should work fine in the likes of SAS/C, GCC, StormC.
The ROM Kernal manual has examples of how to generate GUIs using Workbench intuition, which aren't too bad to get going.
Also check out Aminet, there's lots of good source code examples on there for nearly any task you could imagine.