Benjamin C. Pierce writes:
 > We in the Unison group are also about to embark on a major UI redesign
 > and have been wondering which toolkit to use.  Besides Thorsten's
 > points, there are two more that are critical concerns for us:
 > 
 >    * seamless portability (Unix and Win32)
 >    * ability to build statically linked binaries (this is a pretty
 >      big drawback to the Tk-based solutions)
 > 
 > Comments on these points as well as the others would be very useful. 

There's always QT.  It's nice to work with (at least in C++), very
widely used and comprehensive (because of KDE), and was written from
the start to be cross-platform (X & Win).  NB it is uncontroversially
free these days.