Qt (pronounced "cute") is the UI toolkit produced by TrollTech. It's used in a number of projects, most notably KDE. It is also popular in the commercial world, as code written to use it will compile and work under Windows, Linux, and Mac OS X.
The LSB currently includes two versions of Qt. Version 3 was added in LSB 3.1, but is now deprecated. Version 4 was added as an optional module in LSB 3.1, and was promoted to a required module in LSB 3.2.
As of LSB 3.2, our specification reflects Qt 4.2; we are considering a uplift for LSB 4.0.
In addition, some parts of Qt are not in our spec: