A good (and free) VCL GUI alternative

Question

I've got a project with a rather messy VCL codebase built on Borland C++ Builder 6. I intend to rewrite most parts of it since it's hardly maintainable in it's current state. I'm looking for a good and free alternative to VCL. It is a Windows-only closed source commercial project.

So main requirements are:

1. Free for commercial closed-source projects
2. Manage Windows GUI. Other components (db links and stuff) not required
3. Be extendible (so one could write my own GUI components based on existing ones)
4. Be STL-friendly.

Please post your suggestions, with pros and cons if possible.

EDIT

Thanks for all the answers. I've decided to go with Qt as it has some other nice features like Qt Linguist translation suite.

Answer 1

Try Qt. Its LGPL so it can be used in closed source software. It provides widgets, networking functions, database access, web rendering via WebKit, animations and many more. Its documentation is one of the best of its kind.