There's an interesting Firefox 3 add-on called Pencil, can be used for quick interface sketches (not on the tablet ):

http://www.evolus.vn/Pencil/Home.html

wow! that's pretty cool looking...

Although... is it really faster than Python + GTK ? That's about as rapid as it gets for prototyping GUIs

I think I'll go check it out

python +gtk? is that a serious statement? Is there a way to do quick mockups of GUI screens or are you just stating that python is good for rapid development?

Thanks Bundyo, THIS ROCKS!!!!!! I'm getting teary-eyed already. I've spent so many hours fumbling with MSPaint and Powerpoint to mock up interfaces. This could be a life saver for me. Well, at least I won't have to bring so much work home anymore.

Qt's Designer is also a good way to prototype user interfaces. Even better, you can actually USE the result in your program, i.e. the result IS your user interface.

Graphics dudes use Designer to create awesome and easily usable interface -> send resulting .ui to development team to connect the ui to the other parts of the app.

What can be better than this?

I know.. I am biased...

QT Designer, Glade, Gazpacho, VS and Delphi in Windows - they all have one problem - they only can do the interface stuff.

The Firefox plugin is good at adding additional elements like arrows and boxes and is not that big and heavy as the rest. It can also do several interface types at once and can be easily extended with more elements (with XUL and JS). Also can be kinda good of prototyping web interfaces (though simple ones). Which of the tools can do this?

(EDIT: There's rotate function too)

I need something with a short learning curve, zero cost, and doesn't require admin privaleges in XP to install. Pencil is awesome. I contacted the author about creating new stencils and extensions. There will be more information added to the web page soon.