Ultimate++ is a GUI toolkit with its own IDE, GUI designer, build system, collections, and more. In the recent past it has improved its integration into the operating system with its Chameleon technology for native look and feel.
Unfortunately at the moment it is not 100% geared to be used as a library and you have to use the supplied IDE and development tools to use Ultimate++. Someone has been working on it though and maybe there will be a full released solution soon. If this does come to fruition it will be seriously interesting.
I had a quick read through the Ultimate++ forums recently and came across some interesting things that would make it a more desirable GUI toolkit to use.
First up you have an Office 2007 look skin. This won't be the full ribbon-based UI or anything but it should be interesting to see how it looks and maybe be extended.
Secondly there is the docking toolkit. This looks like a fairly comprehensive docking kit written very quickly. Some of teh examples even had the HUD style docking of Visual Studio. Also, if this does come to fruition this is a very important feature for people evaluating the system.
This toolkit will certainly be moving very fast in teh near future and if these features are suitably polished then it may look like a competitive toolkit to use.