Seconding what Cornstalks said, Qt the cross-platform windowing API is dual-licensed under LGPL, which means you can use it commercially under the LGPL's licensing requirements. You don't need to license your own code under LGPL either.
You're basically safe with LGPL if you: Link to Qt as a DLL (instead of statically linking) , don't modify Qt itself , distribute the Qt license with your project  and link to where others can download Qt .
I am not a laywer, so take my misunderstandings with a grain of thionite:
 I think FSF is working this ambiguity out, but until they do, it's a legal liability unless you sublicense your code under LGPL as well.
 Unless you clearly state you are using a modified version, and you make sure the modified sources are available and you license the Qt modifications under LGPL.
 You aren't licensing your project under LGPL, you are saying the Qt part is licensed under LGPL. Make sure you make that clear to end-users so they aren't confused.
 You are required to make LGPL licensed software (Qt in this case) source code available so others can recompile the DLLs. In practice, just linking to the Qt website is fine.
It's really one of the best open source licenses that permit commercial usage - only really liberal licenses like MIT/zlib/PNG license are better, from a commercial developer standpoint.
[Edit:] Qt used to not be free on Windows, but has been LGPL'd since 2009. Late last year, the previously corporate-managed API has now been transferred over the a community-run management structure with corporate assistance and backing. It's LGPL and permanently and legally irrevocably so - you are safe with it, even commercially. Just make sure you download the framework and IDE from qt-project.org, instead of from the commercially-licensed package that will cost you money (because it comes with commercial developer support).