BlackAdder is growing into a rather nice development environment for Python and PyQt applications. You can use it to rapidly prototype interfaces, or to develop complete, complex applications in a very short time.
The central feature of BlackAdder is the editor, which is specially geared towards working with Python, and includes folding, syntax highlighing and auto-indent. There is also a simple but dependable debugger, a Python interpreter window for when you want to make a quick test, and last, but not least, an excellent gui designer.
Especially the gui designer is worthy of serious attention. It is based on Qt Designer, which is a standard part of the Qt library. It produces designs that can, at your choice, be transformed into executable Python code or compilable C++ code. This means that if you prototype your application in BlackAdder and later, for whatever reason, decide to move it to C++, you can keep all the interface work you've done already.
Using Python and PyQt does not force you to use BlackAdder: you can, if you live in the Unix world, use the free, GPL, version of Qt, which includes the original Qt Designer, the free version of PyQt and Python, to create the same applications. On Windows or OS X, you can use the non-commercial version of Qt with the free PyQt binaries - these cannot be used to develop commercial applications, or in a in-house commercial setting, but are completely identical to the Unix/X11 GPL library in all other respects.
The GUI design files Qt Designer produces and those of BlackAdder are completely compatible. Likewise, using BlackAdder doesn't force you to use PyQt - you can just as well create a Tkinter application with BlackAdder. You won't find much use for the Designer module though, since that only knows about the Qt widgets.
All in all, BlackAdder combines all the tools you need to develop good GUI apps in an extremely convenient package, and the added productivity of this system is well worth the small expense, especially if you intend to develop commercial applications on Windows.