The question should be interpreted from a general point of view and not targeted solely at web apps or desktop apps.
I have been looking around to find a simple and
Real GUI builders are:
While specialized mockup tools are usually:
Almost a decade ago I got frustrated by all of the above and created my own tool: MockupScreens. It became pretty popular quickly :-)
And here is the most complete list of such specialized tools I know of. Many of those are free: http://c2.com/cgi/wiki?GuiPrototypingTools
There's also Balsamiq. I kind of like it, but usually grow tired of these things quite fast. I end up using either pen&paper or OS X's interface builder, which isn't more difficult to use than all these prototyping tools.
Quick and dirty paper prototyping: PowerPoint (see: Powepoint Prototyping Toolkit)
-Great for easily putting together prototypes that can be presented. The slide nature can also serve as a substitute for mock interaction. Downside is lack of standardization. Not for disciplined projects.
Disciplined paper prototyping: Visio
-Standardized and full featured, but more cumbersome
Interactive prototyping: Visual Studio
-Very quick interaction building using drag-n-drop and events. Can be data driven. You can even build a prototype 'base' as a starter kit. Only downside is the temptation to actually make it THE production application. ;)
If you're talking about mock-ups/wireframes (i.e. static pictures) Visio is a tool of choice.
Most software you mentioned is either above the level of the normal business user (i.e. you'll need a specialist to do the mock-ups as opposed to the business users helping you) or are not created for the purpose of mock-ups.
If you need a dynamic prototype then there a plenty of options and everything depends on the type of skills you have available in the team. For example I have a guy who is very strong in HTML. It would be much easier for him to create a HTML page from scratch in notepad that try to do the same thing with Flash in a WYSIWYG tool. Some other people have good Flash skills and could employ them etc.
Expression Blend (http://www.microsoft.com/expression/products/overview.aspx?key=blend) can be used to create quick mockups in XAML. You can store data for the mockup as inline XML in the XAML, or you can quickly convert it to WPF/Silverlight application and build basic business logic behind your mockup using Visual C# Express (http://www.microsoft.com/express/vcsharp/) or Visual Studio 2008.