I just tried Kivy. Thanks for telling me about it! Kivy is pretty awesome: very easy to use, and it Just Works on all platforms, even mobile. It's also painless to deploy Kivy apps to users, which is great.
It's hardware accelerated, so I can write my own custom GLSL shaders for the UI, which is pretty cool. I bet I could even implement a Mario-style game using Kivy. The main application window is just a GL context, and all the UI widget rendering is done via GL.
Unfortunately the Kivy UI seems to be riddled with bugs, and the UI widgets themselves are very alien. For example, the Kivy file browser widget seems to be incomplete (it lacks basic features) and confusing (it behaves nothing like a native file selector). So Kivy is probably too jarring of an experience for users... if I use Kivy in its current form, all my users would balk at the unintuitive UI, since Kivy's widgets don't look or feel anything like native platform widgets. And since users would also be fighting Kivy's UI bugs, I predict every user would immediately ragequit my app. (E.g. drag-selecting text within multiline textboxes seems to be broken, etc.)
That said, Kivy is very close to being an effective way to effortlessly write crossplatform GUI apps. The main reason to use Kivy is because it's so easy to write useful apps with it. I wrote one in about 15 minutes from start to finish. The API is a pleasure to use, and the data binding design is especially brilliant.
Kivy seems pretty fantastic for writing internal tools. Data visualization via Python is now a cinch thanks to Kivy. For example, it'd be easy to write an app that shows the current status of all your server instances. And of course at that point it'd be easy to extend your app's functionality with features like "right click on a server instance to spawn an SSH session into it". Stuff like that is a breeze thanks to Kivy.
Time to try out everyone else's suggestions. Up next: Clojure + Seesaw. (This thread has been incredibly helpful... So many interesting ideas. Thanks, everyone!)
Kivy excels at a lot of things, but I'd say that its real value shows when you want to quickly make a cross-platform mobile app that also happens to run on desktop OSes. The widgets and markup language (KV) are designed to be used in a multitouch environment, and they seem pretty alien when used with a mouse and keyboard. They are pretty intuitive when used on a tablet or phone, though -- I recommend you try compiling your app for Android before giving up on the framework.
That being said, even when compiling for PC it's worked very well for me when a client wants an internal tool and is willing to give up a native feel in exchange for fast (cheap!) development. I wrote this for UCLA a few months ago, and I think the whole thing only took me about 20 hours: https://github.com/sbrother/wassum-lab-lick-program/
I have had similar experiences with Kivy. I previously developed a simple game with PyMT[0] relatively easily, so I was very excited when Kivy came out. I had another project requiring multitouch support on a PC recently and decided to use Kivy and it immediately crashed my window manager every time I tried to run it which turned me off pretty quickly.
I ended up doing the multi-touch UI in JavaScript and HTML instead. It felt much clunkier to work with, but I wasn't comfortable with wrangling Kivy's instability for the rest of the project.
