There are a few alternatives out there, like QT4 (GPL license) and wxwidgets (Free also), but maybe you want something else?
Without paying for QT, it is definitely not compatible with the Boost license (the free version is GPL), but I don't believe there would be problems since this is just going to be an add-on program for use with the Boost libraries, and not a library itself. The source code would just need to be included with it. There shouldn't be any issues with people using this front end for their proprietary, closed-source software. Correct me if I'm wrong. small update:
wxwidgets license is more free than LGPL (does not restrict commercial use anyhow)
http://wxwidgets.org/about/newlicen.htm
I don't even bother to check what is the license for WinAPI, but perhaps an API from ReactOS would do? (in case if you wanted to stick to WinAPI compatibile GUI toolkit).
if you asked me, I would vote for QT4, but that's only my personal opinion.
I also vote for QT. I like the API more. Of course, you could always build a minimal web server (or write some CGI scripts and run from Apache) and do the interface in HTML and Javascript. Jeremy Pack