Qt Licensing

[fullmetalcoder]

You've not read carefully docs/Trolltech site... Qt Open Source Edition is available to Open Source developers for use in Open Source project. As long as you release the full source code of your application (which does not prohibit selling but it is unlikely you'll sell many copies this way...) you may use Qt without purchasing a commercial license. There is no other way around...

The difference between using and modifying GPL-ed software is that derivatives (that is modified software) must hold proper explanations about what has been modified.

LGPL is another Open Source license, actually a more permissive version of GPL. You can release Qt-based software under that license (have a look at Qt GPL exception for more informations) but Qt4 is NOT licensed under it.

[ucomesdag]

When selling open source software, do you only have to enclose the source code for those who actualy buy the software or to everyone who claims it?
And is it a violation of your rights if people use and distribute your software without paying (even in the case you used/included someone else's code)?

[wysota]

When selling open source software, do you only have to enclose the source code for those who actualy buy the software or to everyone who claims it?

Everyone who requests it.

And is it a violation of your rights if people use and distribute your software without paying (even in the case you used/included someone else's code)?

No, if the software has been licenced to allow it (like GPL or LGPL do).