There are many resources on the internet providing Qt information. This is an attempt to mention some of them. Please contact me if you feel that I have missed something.
First out is the official Qt documentation. As reference, this is the best documentation I have ever seen. Crosslinks, even from the examples and great, in-depth descriptions. Make sure not to miss the detailed descriptions that can be found from the "more"-links in the beginning of each class description.
Then, there is the qt-interest mailing list. Most questions gets answered there. Before asking, try google and also, search the list archives.
For those of you interested in the new version of Qt, there is the Qt 4 Resource Center.
Ariya Hidayat has composed a Qt Newbie's FAQ at his site. He has also got a Qt tutorial in Indonesian for the interested.
For in-depth discussions, look at the Qt Quarterly. A quarterly newsletter from the Trolls.
Other in-depth information sources are Justin Karneges' article on signal-safety and the KDE developers list of common mistakes.
The Windows users out there will be happy to know that the old, now unsupported, non-commersial Qt 2.3 is available at the TrollTech ftp site.
The successful KDE project uses Qt as a core component. They both provide a great IDE (KDevelop) and many developer resources. When discussing KDE, I would also like to point to a KDE Tutorial.
There is the QtCentre.org discussion site. It is supported by Trolltech and usually gives quick replies to most questions.
A French speaking site dedicated to Qt.
Those of you reading simplified chinese with an interest of OpenGL, there is a localized OpenGL with Qt tutorial.
For those of you who are interested in seeing how all the widgets look on the different platforms, there is the Qt Widget Gallery, hosted here at digitalfanatics.org.
For 2D sientific plotting there is Qwt, Qt Widgets for Technical Applications. There is a chapter in this tutorial introducing it. If you require 3D plots, look at QwtPlot3D.
For those of you who want to see a simplewindow manager (for X11) written using Qt, check out QLWM.
An interesting application is the VTK Designer. It handles projects and has multiple views of the documents, and can be used to create visual models.
A Qt based screen saver implementation can be found here (english docs).
There are three forks that I wish to bring up: GPLed Qt for win32, Qt without the GUI classes and Qt for DirectFB.
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