Where to find 3rd Party/Open source Qt Widgets applications

[d_stranz]

There is "linux-apps" (formerly qt-apps). Drill down into the Qt Programs -> Qt Widgets area from the menu. There are also Qt-based apps scattered in other sections from which you might be able to extract useful widgets.

qinclude has a huge number of Qt-based tools and libraries.

And KDE is entirely built using Qt.

You can go mining in any of those projects for a wealth of useful things. In some cases, a standalone widget is easy to pull out, other times it is all tied up with other dependencies, which you may or may not want to drag into your project. Good libraries shouldn't really require you to drag everything in just to use a small part of it, in my opinion.