Trolltech has released a tool called "Visual Studio add-in" for their LGPL and GPL release of Qt. They state that the commercial version comes with a full Visual Studio Integration tool but I don't see any description of the differences between these.
What's included in the full version that isn't in the add-in?
The Visual Studio add-in does not work with the Windows Open Source Qt binary installer. To get it to work, you'll have to download the source package and build in manually. The Open Source Windows binary works with the MinGW compiler.
Here's how to add Visual Studio integration using an Open Source build.
The commercial version comes with full Visual Studio integration without the extra steps.
One difference I have noticed (tried the evaluation of the commercial version) is that it have an embedded forms designer, while the free add-on launches QtDesigner for .ui files.
Qt Visual Studio Integration allows you to get quickly from a button on your form to the corresponding button-click-handler (in your C++ source code), by clicking on that widget. For example, if you have a button called "pushButton_42" on your form, click within the embedded Qt form editor onto this button to get to the following function in YourForm.cpp:
void YourForm::on_pushButton_42_clicked()
If the function doesn't yet exist, VS Integration will create it for you. Moreover, VS Integration allows easily connecting C++ code to signals from any widget on your form, by right-clicking on the widget, and selecting the signal you need from a dropdown-list. These features are not available from the "Visual Studio add-in".
Note: I compared Visual Studio Integration version 1.4.3 to add-in version 1.0.1.