Official Documentation - The table of contents for the official documentation for Qt 4.
Porting from Qt 3 to 4 - The official porting guide to convert your Qt 3 applications to Qt 4.
Tools Documentation - The documentation for the tools shipped with Qt 4.
All Overviews - An index of all official Qt 4 overviews and howtos.
A Qt Blog - news and gossip about Qt and Qt related software.
QDevBlog - An aggregation of the blogs of some of the Qt developers.
Mockup - Mockup is a community-based open source desktop operating system built on Qt 4 technology. This site also shows how to compile DBUS support into Qt 4.
QtCentre.org - QtCentre is the best Qt and KDE related forum on the internet.
qtnode.net - QtNode is the base of the #qt irc channel on freenode. The Wiki contains lots of goodies.
The Independent Qt Tutorial - This site also provides the Independent Qt Tutorial for Qt 3, an example driven tutorial based on several chapters with real working code and in-depth discussions about different parts of Qt. The Qt 4 edition of this tutorial can be found in the header links of this page.
The Qt Widget Gallery - This site also provides a gallery of different styles with most Qt 3 widgets, example code, and links to relevant documentation.
I'm always looking for more links - visit the contact page if you would like to contribute!