Qt is licensed under GPL v3 and LGPL v2.1, but you can also buy a license if you want to use and/or modify Qt in a project without having to redistribute the whole thing under GPLv3 or redistribute your modified Qt under LGPL v2.1.

There's actually a lot of closed-source software you know of that uses Qt in some form: Autodesk Maya, Google Earth, Skype, and Spotify are just a few.

I'm not sure whether all of these teams use the commercial version (some may use e.g. the LGPL version) but it's probably not zero.

http://en.wikipedia.org/wiki/Qt_(software)#Software_using_Qt