Recommendation for GUI lib?

[Jan Erik Moström]

I want to write a few programs with a GUI but I don't want to use Tk. 
Instead I'm looking for some other library, I've tried to look around 
and also asked a few but I don't really know what to use.

Do you have any recommendations? Primary platforms are OS X and Linux.

I, of course, want to have "standard" widgets but a "calendar 
view"/"date picker" is a plus.

= jem