Was reading the main page of the project and it said:
>GIMP (and GTK+) for Win32: Win32 native support of GTK+ (mainly 2.x), supported by Tor Lillqvist (tml), using MS VC or MinGW as compiler. Their work is integrated with the mainline gnome development. And many GTK+ based applications use it to provide their Win32 port, such as GIMP, Sylpheed-Claws, Gaim, etc.
Just had to comment on the Sylpheed-Claws application. I've tried building it myself under mingw and msys using GTK+ version as mentioned above. My opinion was that it was not yet ready for prime time. The project offers a precompiled Windows version, but it was an earlier version than the one I tried to compile and I also found it buggy. If you want a mail program that will work on both Windows and Linux and is similar to Sylpheed Claws, then I highly recommend you go with the original not the fork. Sylpheed is available from http://sylpheed.sraoss.jp/en/ I've had no problems using it on Windows and it also uses GTK+ libraries.
Other good programs that I've been able to build with GTK+ using mingw and msys or that are already built and available are searchmonkey, stardict, sdcv and SciTE. I'll stop there, but there are several other applications that will build with mingw/msys/GTK+ natively on Windows.