Which GUI API is
-easy to learn
-preferably portable, otherwise suitable for Linux
-has well explained and comprehensive documentation/tutorials
-has a decent, helpful forum.
Please don't say Qt because the last two don't apply to it.
Sorry, but Qt.
Qt is easy to work with and has a decent documentation. Using Google you'll find the appropriate part of the Qt documentation for a certain problem you have very quickly. Qt Creator has a decent GUI designer that makes things easier.
And there are lots of forums where you can get help with Qt, you don't need to post in the official one.
Aside from that there is gtkmm, but the gtkmm documentation had me tearing my hairs out before - something that I planned to complete in ten minutes took several days in the end (a simple list with several columns). That was the only exception though, I found the rest of gtkmm to be fairly easy to pick up.
wxWidgets is another popular candidate, which I haven't used before.
I'd say that all three ( Qt, wxWidgets and GTKmm ) are quite easy and you can find easy tutorials / documentation for them
wxWidgets may be more portable since it's a wrapper of other toolkits ( this will make wx programs look somewhat native on any platform )
1 - Very easy to learn and use. Its documentation is probably the best you can find, and once you learn how to use a class it's easy to guess how to use another without having a deep look at the docs. This coherence is its strongest point.
2 - Very Portable, I write applications targetting Windows, MacOsX (mainly) and Linux (sometimes) and it is nice to have just a code pool to maintain. Qt use native widgets in every platform, so your applications will look and feel natively (very important for the MacOs X platform)
3 - Works in embedded devices, using just a bare framebuffer. It is highly costumizable and you include just what you need, every byte matters. It is Nokia's preferred choice.
4 - Qt Creator is a very decent IDE.
5 - It is a framework, not only a collection of GUI classes, but other useful stuff is there, like networking and threading classes.
6 - bugs are fixed extremelly quickly.
I have used other solutions but Qt is far ahead of all of them, but latelly I have heard of very good progress of wxWidgets. I used it when it was called wxWindows, and to be honest was just a widget toolkit these days, maybe has progressed further.
Thanks alot for such detailed responses!
I'm trudging along with Qt - i can't deny that it is better than all the other GUis that i've looked at but i do wish their tutorials were a bit better.
http://zetcode.com/tutorials/qt4tutorial/
good tutorial, it got me started about a week ago and I'm pretty happy with what I have learned by now.
(I also reread the classes and inheritance stuff in the c++ tutorial, it was a while ago I had read it and it helped me a lot)
I have a good experience with Qtcentre.org. Also qtforum.org showed up numerous times when I searched google.
(maybe it is of no use for you anymore, but it can help others)