A bit less heavyweight, but I've heard
WxWidgets is a good C++ library for cross-platform GUIs. It's used by
Filezilla and quite a few other programs that tend to look nice, everywhere. It looks to me at first glance to be quite a nice size for a program like SWIFT.
It's worth pointing out that the future of Qt is a bit shrouded in mystery at the minute as it's primary developer, Nokia, has recently committed to a Windows-y future. Qt is certainly not going anywhere - it's huge and used all over the place - at the very worst, I'm sure it would be folded into the KDE guys domain, but it's worth pointing out that there may be some changes in the landscape. Not that I'm suggesting there's any risk with starting a project with it. Just one to watch the evolution of ...
Personally, I'd be reluctant to commit to .NET as a cross-platform solution - Mono is always many versions behind and I don't think Microsoft will ever commit their own resources to develop simultanous versions alongside the Windows releases. Even if they did, there's so much distrust of them based on their own history that - rightly or wrongly - it still wouldn't be likely to get much use in the open source world. If you were going to try out C# and you wanted it to be cross-platform, I think you'd have to try really hard to stick with just the Mono tools and not let the Windows-only .NET extensions etc. creep in.
I'm sure me, Dave J, maybe MartinW and others would be willing to try compiling it on other platforms.
Sam.