An Open Source C++ GUI library.

SmartWin++ is a 100% free C++ GUI and SOAP library for developing Windows applications both on Desktop, Pocket PC, Windows Mobile or Windows CE based systems, it's free both as in "free beer" and as in "free speech", you can freely use SmartWin++ for commercial applications and for Open Source applications thanx to its BSD license!
SmartWin++ was initially made to replace MFC and WTL but today thanx to WineLib it can also be used on non-Windows systems. SmartWin++ is a modern designed C++ library, it uses STL where possible and boost where needed, but if you download SmartWin++ you don't need to bother downloading boost, everything you need is bundled in the download except for the C++ compiler.
Good compilers can be found for free either at Microsoft (Visual C++ 2003 Toolkit) or at Bloodsheds (Dev-C++).
If you're using Dev-C++ everything you need is bundled with the compiler as long as you download the version bundled with MingW/gcc 3.4.2, if you want to use the VC Toolkit you need to also download the Windows Server 2003 SP1 Platform SDK.
If you're downloading Dev-C++ you also get a free IDE and you'll probably easier manage to configure everything then by doing everything "by hand" with the VC++ Toolkit. If you're determined to still use the VC++ Toolkit you might want to have a look at Eclipse which is a free IDE which can be configured to use the VC++ Toolkit, though here you're on your own...
Of course if you're owning a full copy of either Visual Studio C++ 7.1 or Visual Studio C++ 2005 you're home free and get to use the full IDE!
Last but not least I'd like to especially put forth Sally - A Simple C++ IDE which is an IDE especially made to support e.g. SmartWin++, Sally is a truly impressive IDE and gives you among other things true WYSIWYG and RAD capabillities for your SmartWin++ projects!
Though Sally is still in its early Beta stages it's definitely worth a check!
SmartWin++ officially supports these compilers:

* Microsoft Visual C++ 2003
* Microsoft Visual C++ 2005
* MinGW/gcc 3.4

We're also working heavily on Wine support at the moment and have actually made it really far in this area. Take a look here if you'd like to see how this work is going! Some of these compilers can also build applications for other platforms like for instance Visual C++ 2005 which can build Windows CE/Pocket PC/Smart Phone/Windows Mobile applications etc. We fully support building Windows Mobile and Pocket PC applications on VC++ 2005, and probably it will work building Smart Phone and Windows CE based applications utilizing SmartWin!

It will probably work with any newer versions of both VC++ and gcc, though since SmartWin++ uses some rather advanced C++ concepts it stresses the compiler to the edge of the C++ standard, this excludes older non-conforming compilers like for instance Codewarrior and Borland.
If you want to have more background information about SmartWin++ I suggest you browse around this website and especially look at our wiki since that's where you'll find the most information about SmartWin++.

The domain is sponsored by Frost Innovation the FREE ASP.NET Ajax Widgets vendor.

Happy SmartWinning!