Do you need it to be cross-platform? Otherwise you could use Cocoa directly. Although it currently is a bit of a pain to do so.
I don't care about cross-platform compatibility for now. I've found dstep, but haven't looked into it yet. It does look like it could be somewhat painful to use, though, especially considering that I'm not too comfortable with ObjC.
I also don't really care for how pretty it looks at the moment. Right now it's for myself, so I just want something that lets me build a GUI in a relatively straightforward way, even if the result is a bit on the ugly side. Obviously, if what I'm doing goes anywhere, I will look into other options, but I need whatever I can get to work to start with.
