I'd like to make a simple GUI that offers buttons which I can drag and drop into other Windows applications such that this other applications receives a certain string depending on the button chosen.
What would be the easiest GUI framework for Python that allows this drag and drop?
Theoretically, you can use any library for which a graphical drag-and-drop designer exists. Such tools often generate markup language which the library parses, and sometimes they generate code directly. The latter is language-dependent whilst the former shouldn't be. Either way, you'll find a way of doing it with Python.
Practically, some libraries have better visual designing tools that others. WinForms designer was really elegant and seamless when I used it, so maybe IronPython? How about PyGTK, with Glade, or the aforementioned PyQt? Maybe Jython using Swing designed in NetBeans?
EDIT: Whoops, I didn't read the question properly. What you're looking for is a framework with drag-and-drop capabilities, i.e. most of them. There's lots of things to consider though, for example are the destination and source windows going to be from the same process? Will they be written with the same framework? These things could be relevant or not, depending on how it's all written.
Simple GUI Windows Drag&Drop