Qt includes a set of examples that cover nearly every aspect of Qt development. They aren't meant to be impressive when you run them, but in each case the source code has been carefully written to illustrate one or more best Qt programming practices.
You can run the examples from the Examples and Demos Launcher application (except see QML Examples for special instructions for running those examples).
The examples are listed below by functional area. Each example listed in a particular functional area is meant to illustrate how best to use Qt to do some particular task in that functional area, but the examples will often use features from other functional areas as well for completeness.
If you are new to Qt, you should probably start by going through the Tutorials, and then begin with the Application example.
In addition to these examples and the tutorials, Qt includes a selection of demos that deliberately show off Qt's features. You might want to look at these as well.
These examples are provided under the terms of the Modified BSD License.
Examples by Functional Area