On my trawl through GUI toolkits and my general web browsing I have encountered the name Juce from time to time. It's not just a GUI toolkit, it aims to be as comprehensive as possible.
What stopped me from taking a closer look in the past is that it is a non-native UI, but hey, nothing ventured...
It provides excellent support to essentially a vector graphics based user interface. It seems really solid and smooth. The library itself was developed for the audio sequencer application Tracktion, and looking more closely at this you can see where the library's real power lies. Providing you don't want a platform standard GUI toolkit UI you are away, like if you wanted a midi sequencing apllication to look like a keyboard not a standard windows dialog based application. I'm sure it must be possible to apply a Windows XP/Vista theme with enough effort.
Juce is also on the Wikipedia which covers the main points from the website. I recommend downloading the example application the Juce website as it is interesting seeing how it behaves. I do find it all a little blurry, but I think that is the theme supplied not the toolkit itself.
