Need an easy and cross-platform way to handle audio and video? Phonon's API will be very familiar to Qt developers and supports multiple backends, including GStreamer, VLC, and anything else you might want to develop.
If you need a privacy-respecting way to display maps and even the whole world, Marble provides data models and widgets to make that a walk in the park.
If you need libraries for handling email, events, and more, the KDE PIM libraries have withstood not only the test of time but also the test of enterprise customers.
