Currently, we have:
OpenDialog
SaveDialog
GPToolDialog
MessageBox
But there's no way to actually get user input. Why is this? Does ESRI plan on eventually providing a way to get user input?
Check out ArcMap and Python: Closing the VBA Gap | Esri Video
Using wxPython as described in this video works even if it isn't officially supported by esri.
But be cautious. As per my other post about Tkinter, wxPython GUI's in Python add-ins can randomly cause ArcMap to crash. A very simply GUI may not experience the problem, but I would not trust it. My experience with wxPython and several other Python GUIs in a Python add-in all had the same problem (by my GUI was somewhat more complicated).
I then found that other people had concluded that non-ArcMap GUIs in Python addins are incompatible with ArcMap and likely to cause it to crash.