Well to my knowledge, the only built in package is Tkinter. It's a really an ugly UI out of the box, but I've found if you use it with the ttk module, you can make some relatively modern looking Apps. Unfortunelty, there's no visual interface for building Apps using these modules, you'll probably end up building your interfaces through code.
In my experience, ArcMap doesn't play well with Tkinter when trying to launch a custom GUI from a Python Add In. When trying it in 10.2, ArcMap immediately crashed without even an error window. However, I found if you launch your GUI through a separate Python file by calling on it using the subprocess module, the custom Python GUI in ArcMap works great! If your GUI takes in parameters that is needed to pass back to the main Python AddIn file, you can pass it back as text through the subprocess stdout.
If you're interested in how to run a Tkinter GUI through the subprocess module, I can dig up a code example for you.
I know there are some other libraries out there that offer alternative Python based GUIs, but since they're not built in, my experience has mostly been Tkinter based.
Hope this helps!
